The neonatal Fc receptor (FcRn) is involved in IgG metabolism and transport in placental mammals. However， whether FcRn is responsible for IgG transfer from maternal serum to colostrum/milk is controversial. Interestingly， large domestic animals， such as cows， pigs， sheep， and horses， in which passive IgG transfer is exclusively completed via colostrum/milk， all express an FcRn α-chain that is shorter in the cytoplasmic tail (CYT) than its counterparts in humans and rodents. To address whether the length variation has any functional significance， we performed in vitro experiments using the Transwell system with the MDCK cell line stably transfected with various FcRn constructs; these clearly suggested that truncation of the CYT tail caused a polar change in IgG transfer. However， we observed no evidence supporting functional changes in IgG in vivo using mice in which the FcRn CYT was precisely truncated. These data suggest that the length variation in FcRn is not functionally associated with passive IgG transfer routes in mammals.