Identification of the Ped gene at the molecular level: the Q9 MHC class I transgene converts the Ped slow to the Ped fast phenotype

The Ped (preimplantation embryo development) gene, which maps to the Q region of the mouse major histocompatibility complex (MHC), controls the rate of cleavage division of preimplantation mouse embryos and subsequent embryonic survival. Of the ten known MHC class I genes in the Q region of the mouse MHC, four--Q6, Q7, Q8, and Q9--are almost identical and encode similar proteins, all called the Qa-2 antigen. Previous studies have suggested that the Q9 gene encodes the Ped gene. To test this directly, one-cell embryos from the CBA/Ca strain (Ped slow) that is missing the Q9 gene and the Qa-2 antigen were injected with the Q9 gene from the C57BL/10 strain (Ped fast) that possesses the Q9 gene and expresses the Qa-2 antigen. The resulting Q9 transgenic mice were found to express the Qa-2 antigen. In addition, it was found that introduction of the Q9 gene converted the Ped gene phenotype of the recipient strain from slow to fast. Therefore, the Ped gene product is the Qa-2 antigen encoded by the Q9 gene.