In order to distinguish our injected mouse MC3T3-E1 cells from the endogenous mouse osteoblasts, we exploited the knowledge that homozygous Nu/Nu mice have a spontaneous mutation in the Forkhead Box N1 (FOXN1) gene (resulting in hairlessness and athymia), and thus are deficient for FOXN1 [61–63]. This evidence concerns the gene FOXN1 and Aplasia of the thymus.