diff --git a/test/test_cluster.py b/test/test_cluster.py
index 72e0b99..2f16d6d 100644
--- a/test/test_cluster.py
+++ b/test/test_cluster.py
@@ -5,6 +5,7 @@
 from tspyro.cluster import make_clustering_gibbs
 from tspyro.cluster import make_fake_data
 from tspyro.cluster import make_reproduction_tensor
+from tspyro.cluster import transpose_sparse
 
 
 @pytest.mark.parametrize("num_epochs", [1, 10])
@@ -16,7 +17,10 @@ def test_clustering(num_variants, num_samples, num_clusters, num_epochs):
     data = make_fake_data(num_samples, num_variants)
 
     print(f"Creating {num_clusters} clusters via {num_epochs} epochs")
-    clusters = make_clustering_gibbs(data, num_clusters, num_epochs=num_epochs)
+    assignment, clusters = make_clustering_gibbs(
+        data, num_clusters, num_epochs=num_epochs
+    )
+    assert assignment.shape == (num_samples,)
     assert clusters.shape == (num_variants, num_clusters)
 
     print("Creating a reproduction tensor")
@@ -33,6 +37,17 @@ def test_clustering(num_variants, num_samples, num_clusters, num_epochs):
     assert torch.allclose(reproduce.sum(-1), torch.ones(num_clusters, num_clusters))
 
 
+@pytest.mark.parametrize("num_samples", [5, 10, 100])
+@pytest.mark.parametrize("num_variants", [4, 8, 100])
+def test_transpose(num_variants, num_samples):
+    data1 = make_fake_data(num_samples, num_variants)
+    data2 = transpose_sparse(data1)
+    data3 = transpose_sparse(data2)
+    for k, v1 in data1.items():
+        v3 = data3[k]
+        assert (v1 == v3).all()
+
+
 if __name__ == "__main__":
     parser = argparse.ArgumentParser(description="benchmark clustering algorithm")
     parser.add_argument("-n", "--num-samples", default=1000, type=int)
diff --git a/tspyro/cluster.py b/tspyro/cluster.py
index a900aa4..fdb6351 100644
--- a/tspyro/cluster.py
+++ b/tspyro/cluster.py
@@ -1,3 +1,5 @@
+from typing import Tuple
+
 import numpy as np
 import torch
 import tqdm
@@ -33,6 +35,9 @@ def check_sparse_genotypes(data: dict):
     assert index.shape == (int(offsets[-1, -1]),)
     assert index.shape == values.shape
 
+    assert offsets[0, 0] == 0
+    assert (offsets[1:, 0] == offsets[:-1, 1]).all()
+
     return offsets, index, values
 
 
@@ -66,6 +71,43 @@ def make_fake_data(num_samples, num_variants):
     return data
 
 
+def transpose_sparse(data: dict, num_cols=0) -> dict:
+    """
+    Convert from row-oriented to column-oriented.
+    This function is its own inverse.
+
+    :param dict data: A dict representing sparse genotypes.
+    :param int num_cols: Optional number of columns.
+    :returns: A dict representing sparse genotypes.
+    :rtype: dict
+    """
+    old_offsets, old_index, old_values = check_sparse_genotypes(data)
+    num_rows = len(old_offsets)
+    num_cols = max(num_cols, 1 + int(old_index.max()))
+
+    # Initialize positions and create offsets.
+    position = torch.zeros(1 + num_cols, dtype=torch.long)
+    for r in range(num_rows):
+        beg, end = old_offsets[r].tolist()
+        index = old_index[beg:end]
+        position[1 + index] += 1
+    position.cumsum_(0)
+    new_offsets = torch.stack([position[:-1], position[1:]], dim=-1)
+
+    # Populate index and values.
+    new_index = torch.zeros_like(old_index)
+    new_values = torch.zeros_like(old_values)
+    for r in range(num_rows):
+        beg, end = old_offsets[r].tolist()
+        index = old_index[beg:end]
+        values = old_values[beg:end]
+        new_index[position[index]] = r
+        new_values[position[index]] = values
+        position[index] += 1
+
+    return dict(offsets=new_offsets, index=new_index, values=new_values)
+
+
 def naive_encoder(ts):
     """
     Make an encoding of the sparse genotype data naively: haplotype by haplotype.
@@ -145,7 +187,7 @@ def make_clustering_gibbs(
     num_clusters: int,
     *,
     num_epochs: int = 10,
-) -> torch.Tensor:
+) -> Tuple[torch.Tensor, torch.Tensor]:
     """
     Clusters sparse genotypes using subsample-annealed Gibbs sampling.