A novel method for mapping a modifier gene that is syntenic to its major gene was used to map a male-sex-limited modifier of the expressivity of the Compact phenotype in the myostatin mutant (Mstn(Cmpt-dl1Abc)) Compact mouse. The modifier was mapped to the general region of D1Mit262, 40 cM distal to Mstn on chromosome 1. Myogenin, a postulated downstream target of myostatin, maps to the same region.