Genetics
Nude mice have a spontaneous deletion in the FOXN1 gene. (Humans with mutations in FOXN1 also are athymic and immune deficient.) Mice with a targeted deletion in the FOXN1 ("knockout" mice) also show the "nude" phenotype. Since nude females have underdeveloped mammary glands and are unable to effectively nurse their young, nude males are bred with heterozygous females.
Read more about this topic: Nude Mouse