diff --git a/malariagen_data/util.py b/malariagen_data/util.py
index 7372620e3..984102d0c 100644
--- a/malariagen_data/util.py
+++ b/malariagen_data/util.py
@@ -664,17 +664,71 @@ def _valid_contigs(resource):
 
 
 def _parse_single_region(resource, region: single_region_param_type) -> Region:
+    """The function to parse a single region specifications into a Region object
+    
+    Accepted formats:
+    - Region instance: Return as it is
+    - Mapping with keys: Fetching key and values and return a new Region object
+    - String: 
+        - contig name (e.g. "chr1")
+        - genomic interval (e.g. "chr1:1000-2000")
+        - feature ID (if genome_features available)
+    
+    Raises:
+    -TypeError if input type is invalid
+    -ValueError if region cannot be parsed or is invalid"""
+
     if isinstance(region, Region):
         # The region is already a Region, nothing to do.
+        contig=region.contig
+        start=region.start
+        end=region.end
+        
+        if contig is None:
+            raise ValueError("Region mapping must include 'contig'.")
+        
+        if contig not in _valid_contigs(resource):
+            raise ValueError(f"Unknown contig {contig!r}.")
+        
+        if start is not None:
+            if not isinstance(start, int) or start < 1:
+                raise ValueError(f"Invalid start position: {start!r}.")
+
+        if end is not None:   
+            if not isinstance(end, int) or end < 1 or end > resource.genome_sequence(region=contig).shape[0]:
+                raise ValueError(f"Invalid end position: {end!r}.")
+            
+        if start is not None and end is not None:    
+            if start > end:
+                raise ValueError(f"End position must be greater than start position.")
+        
         return region
 
     if isinstance(region, Mapping):
         # The region is in dictionary form, convert to Region instance.
-        return Region(
-            contig=region.get("contig"),
-            start=region.get("start"),
-            end=region.get("end"),
-        )
+        contig=region.get("contig")
+        start=region.get("start")
+        end=region.get("end")
+
+        if contig is None:
+            raise ValueError("Region mapping must include 'contig'.")
+        
+        if contig not in _valid_contigs(resource):
+            raise ValueError(f"Unknown contig {contig!r}.")
+        
+        if start is not None:
+            if not isinstance(start, int) or start < 1:
+                raise ValueError(f"Invalid start position: {start!r}.")
+
+        if end is not None:   
+            if not isinstance(end, int) or end < 1 or end > resource.genome_sequence(region=contig).shape[0]:
+                raise ValueError(f"Invalid end position: {end!r}.")
+            
+        if start is not None and end is not None:    
+            if start > end:
+                raise ValueError(f"End position must be greater than start position.")
+        
+        return Region(contig=contig, start=start, end=end)
 
     if not isinstance(region, str):
         raise TypeError(
diff --git a/tests/test_util.py b/tests/test_util.py
index 974046ab1..e85056da1 100644
--- a/tests/test_util.py
+++ b/tests/test_util.py
@@ -1,14 +1,123 @@
-"""Tests for Region.__repr__ and CacheMiss.__repr__ / default message."""
-
 import pytest
-
-from malariagen_data.util import CacheMiss, Region
-
-
-# ---------------------------------------------------------------------------
-# Region
-# ---------------------------------------------------------------------------
-
+from malariagen_data.util import _parse_single_region, Region, CacheMiss
+from unittest.mock import MagicMock
+
+# _parse_single_region tests
+
+@pytest.fixture
+def mock_resource():
+    resource = MagicMock()
+    resource.contigs = ("2R", "2L", "3R", "3L", "X")
+    del resource.virtual_contigs
+    resource.genome_sequence.return_value.shape = [1_000_000]
+    return resource
+
+def test_parse_region_contig(mock_resource):
+    r = _parse_single_region(mock_resource, "2L")
+    assert r.contig == "2L"
+    assert r.start is None
+    assert r.end is None
+
+    r = _parse_single_region(mock_resource, {"contig":"2L"})
+    assert r.contig == "2L"
+    assert r.start is None
+    assert r.end is None
+
+    r = _parse_single_region(mock_resource, Region("2L"))
+    assert r.contig == "2L"
+    assert r.start is None
+    assert r.end is None
+
+# String test
+def test_parse_region_interval(mock_resource):
+    r = _parse_single_region(mock_resource, "2L:100-200")
+    assert r.contig == "2L"
+    assert r.start == 100
+    assert r.end == 200
+
+def test_parse_region_invalid_string(mock_resource):
+    invalid_regions = ["invalid_region", 
+                       "3L:abc-100", 
+                       "2L:100-25d", 
+                       "", 
+                       "2R:100-50", 
+                       "3R:0-10", 
+                       "2R-100-200", 
+                       "2L:150"] 
+    for region in invalid_regions:
+        with pytest.raises(ValueError):
+            _parse_single_region(mock_resource, region)
+
+# Mapping test
+def test_parse_region_invalid_dictionary(mock_resource):
+    invalid_regions = [{}, 
+                       {"start": 100, "end":200}, 
+                       {"contig":"3L", "start":-2, "end":10}, 
+                       {"contig":"X", "start":10, "end":-100}, 
+                       {"contig":"2L", "start":100, "end":10}, 
+                       {"contig": "Invalid_contig", "start":10, "end":20}, 
+                       {"contig":"2L", "start":"abc", "end":10}, 
+                       {"contig":"2R", "start":100, "end":"bcd"}
+                       ]
+    
+    for region in invalid_regions:
+        with pytest.raises(ValueError):
+            _parse_single_region(mock_resource, region)
+    
+def test_parse_region_dictionary_only_start(mock_resource):
+    r = _parse_single_region(mock_resource, {"contig": "2L", "start":200})
+    assert r.contig == "2L"
+    assert r.start == 200
+    assert r.end is None
+
+def test_parse_region_dictionary_only_end(mock_resource):
+    r = _parse_single_region(mock_resource, {"contig": "2L", "end":200})
+    assert r.contig == "2L"
+    assert r.start is None
+    assert r.end == 200
+
+# Region instance test
+def test_parse_region_instance_interval(mock_resource):
+    r = _parse_single_region(mock_resource, Region("2L", start=100, end=200))
+    assert r.contig == "2L"
+    assert r.start == 100
+    assert r.end == 200
+
+def test_parse_region_instance_only_start(mock_resource):
+    r = _parse_single_region(mock_resource, Region("2L", start=200))
+    assert r.contig == "2L"
+    assert r.start == 200
+    assert r.end is None
+
+def test_parse_region_instance_only_end(mock_resource):
+    r = _parse_single_region(mock_resource, Region("2L", end=200))
+    assert r.contig == "2L"
+    assert r.start is None
+    assert r.end == 200
+
+def test_parse_region_invalid_instance(mock_resource):
+    invalid_regions = [Region(contig=""), 
+                       Region(contig="", start = 100, end = 200), 
+                       Region(contig="3L", start=-2, end=10), 
+                       Region(contig="X", start=10, end=-100), 
+                       Region(contig="2L", start=100, end=10), 
+                       Region(contig= "Invalid_contig", start=10, end=20), 
+                       Region(contig="2L", start="abc", end=10), 
+                       Region(contig="2R", start=100, end="bcd")
+                       ]
+    
+    for region in invalid_regions:
+        with pytest.raises(ValueError):
+            _parse_single_region(mock_resource, region)
+
+# Region type test
+def test_parse_region_invalid_type(mock_resource):
+    invalid_types = [123456, ["2L:100-230"], ("2L:100-250",), True, False, 3.154]
+    for types in invalid_types:
+        with pytest.raises(TypeError):
+            _parse_single_region(mock_resource, types)
+
+#  Region tests
 
 def test_region_repr_contig_only():
     r = Region("2L")
@@ -32,11 +141,7 @@ def test_region_repr_start_only():
     assert repr(r) == "Region('X', 500, None)"
     assert str(r) == "X:500-"
 
-
-# ---------------------------------------------------------------------------
-# CacheMiss
-# ---------------------------------------------------------------------------
-
+# CacheMiss tests
 
 def test_cache_miss_no_key():
     cm = CacheMiss()