ASIP and hyperinsulinism: For example, mice with the lethal yellow mutation (Ay), a dominant gain of function mutation resulting in ectopic and ubiquitous ASP expression, have a yellow/blonde coat color and are obese with hyperinsulinemia due to binding of the overexpressed ASP to MC4R (Takeuchi et al., 1989; Bultman et al., 1992; Robbins et al., 1993; Duhl et al., 1994a,b; Michaud et al., 1994).