Evaluating the functional relevance of mutant MC4R by ortholog sequence comparison and functional testing. Ala175Thr within transmembrane domain 4 has been associated with obesity and positively tested for dysfunction (59–62). In amphibian, fish, and Western European hedgehog, MC4R Ala175 is naturally substituted by Thr; accordingly, the human Ala175Thr receptor protein has been examined and recently found to possess wild-type functionality (C Stäubert, H Römpler, and T Schöneberg; unpublished results).