Although not being an obesity fish model, studies in energetically divergent rainbow trout lines selected for low (lean line, LL) and high (fat line, FL) muscle adiposity, revealed impaired central leptin signaling system in the FL fish, probably linked to high muscle fat, since no differences in appetite regulation and feed intake between the two rainbow trout lines were found (102). The gene discussed is LEP; the disease is obesity due to melanocortin 4 receptor deficiency.