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Four recently discovered homologues of the brown adipose tissue-specific mitochondrial uncoupling protein (UCP1) vary from 29% to 58% in their similarity to UCP1. Although these homologues share important structural features with UCP1 and like UCP1 can reduce the mitochondrial membrane potential when expressed in yeast, there is no clear evidence that they can function thermogenically in vivo. On the other hand, evidence continues to accumulate indicating that the up-regulation of Ucp1 reduces excessive adiposity.
Kozak et al. (Sat,) studied this question.