Short break (SBK) is a new semilethal mutation of Japanese quail (Coturnix japonica). The SBK individuals are characterized externally by short breaks, shanks, and digits. The shank is also thicker than the wild type. The shape of the mutant beak does not show a parrotlike appearance, contrasting with that of other poultry chondrodystrophic mutants reported in the literature. Bones in the fore and hind limbs of the SBK mutant are also shorter than the wild types. The humerus and ulna in the wing are significantly thicker than the wild types. Other bones in the wing and leg show approximately the same thickness as the wild types. The majority of the SBK mutants die at the late embryonic stage. Some chicks can hatch with no assistance, but almost all die around 3 days of age. However, a part of the SBK individuals (5.3%, 8/151) reach sexual maturity and can reproduce. Genetic analyses revealed that the SBK mutation is controlled by an autosomal recessive gene. The proposed gene symbol is sbk. The sbk gene is allelic to the previously reported stumpy-limb (sl) gene that expresses a somewhat similar phenotype to the SBK.