Mc4rt2aCre/t2aCre Mrap2fl/fl mice fed ad libitum regular chow developed early-onset obesity with significantly higher body weight (Figure 1, A–F) and fat mass (Figure 1, K–R), associated with hyperphagia (Figure 1, S–V) compared with their Mc4rt2aCre/t2aCre Mrap2+/+ littermates. The gene discussed is MRAP2; the disease is obesity due to melanocortin 4 receptor deficiency.