The structure and evolution of PTCH2 in therian mammals. a. Amino Acid alignment of the additional PTCH2 tammar wallaby (Me) exon 21a with sequence derived from intron 21 in humans (Hs). The red arrowhead indicates the location of the premature stop codon (-) at amino acid 35 in humans. Black shading indicates identical amino acids, grey shading indicates like amino acids and no shading indicates divergent amino acids. Numbers indicate amino acid position. b. Schematic of the PTCH2 protein derived from the native human isoform (left) the Δ-22 PTCH2 isoform (middle) and tammar PTCH2 with the additional exon 21a (right). c. Phylogenetic tree showing the evolution of exon 21a in mammals. Exon 21a was present in the common therian ancestor (blue arrowhead). The exon was retained without a nonsense codon in the marsupial lineage (blue arrowhead), was mutated in humans to contain a premature stop resulting in the Δ-22 isoform (red arrowhead) and was lost in the rodent lineage (green arrowhead).