HCRT and obesity due to melanocortin 4 receptor deficiency: In particular, orexin-neuron deficient male mice do not show obesity at least up to 43 weeks of age on the C57BL/6J background [27, 29, 31, 52], whereas they develop obesity at 14–16 weeks of age on mixed C57BL/6-DBA backgrounds [22, 27, 52].