The role of a specific carbohydrate-binding protein, the mannose/GalNAc-4-SO<sub>4</sub> receptor (M/G4SR), in mammalian physiology, specifically in reproduction, was elucidated. An attempt was made to generate the relevant null mutant, but the resultant M/G4SR<sub>-/-</sub> homozygotes died in utero. Thus, attention was turned to the heterozygous state of the receptor gene (M/G4SR-/+).