Rat Atp7a occupied a single open reading frame (27-4502) which coded for a protein of 1492 residues. Rat Atp7a was 98% and 95% identical to published sequences for the mouse and Chinese hamster, respectively, and 94% homologous to human ATP7A. Compared to ATP7A, the rat transcript coded for an additional alanine (A446) in the heavy metal binding (Hmb) domain and showed a 34 bp gap in the 3′ UTR. Based on published sequence data, hydropathic profiles for rat, mouse, Chinese hamster, and human Cu-ATPases were practically identical with the exception of 8 additional amino acid residues between the 4th and 5th Hmb sites in the human. As deduced from amino acid sequence data, Hmb was predicted to have regions with helical and beta structures. All four species had five of the six metal binding sites centered within hydrophobic regions. The comparative analyses suggested that the Hmb region of the molecule could experience numerous amino acid substitutions with no apparent disruption to the ATPase transport function whereas variations to the ATPase domain would be more critical.