diff --git a/src/cool_seq_tool/mappers/exon_genomic_coords.py b/src/cool_seq_tool/mappers/exon_genomic_coords.py
index 20f1d2a..7c68600 100644
--- a/src/cool_seq_tool/mappers/exon_genomic_coords.py
+++ b/src/cool_seq_tool/mappers/exon_genomic_coords.py
@@ -658,6 +658,9 @@ async def _get_alt_ac_start_and_end(
     ) -> tuple[tuple[GenomicAlnData, GenomicAlnData] | None, str | None]:
         """Get aligned genomic coordinates for transcript exon start and end.
 
+        ``tx_exon_start`` and ``tx_exon_end`` will always have transcript and genomic
+        accession.
+
         :param tx_ac: Transcript accession
         :param tx_exon_start: Transcript's exon start coordinates. If not provided,
             must provide ``tx_exon_end``