Absence of a functional gastric acid secreting H+/K+-ATPase in monotremes. (a) Phylogenetic tree showing the distribution of a functional α subunit of the H+/K+-ATPase gene (ATP4A) in vertebrates, indicating in red the absence of this gene in platypus. The percentage of identities at the protein level of ATP4A from human (Homo sapiens), dog (Canis familiaris), opossum (Monodelphis domestica), lizard (Anolis carolinensis), chicken (Gallus gallus), and frog (Xenopus tropicalis) is shown in yellow boxes. (b) Gene structure of ATP4B and amino acid sequence alignment of the indicated exons with ATP4B from different vertebrate species, including the teleost fish stickleback (Gasterosteus aculeatus). Electropherograms and sequence translation of platypus ATP4B exons 3, 4, and 7 showing the presence of premature stop codons and a frameshift (red arrow). MYA, million years ago.