This is because, although the number of active ZP3 molecules in the mouse ZP does not seem to be crucial (Liu, Litscher, & Wassarman, 1995), in vitro experiments with recombinant proteins produced in embryonal carcinoma (EC) cells strongly suggested that O‐glycosylation of ZP3 S332 and S334 (encoded by Zp3 exon 7) was essential for its sperm‐binding function (Chen, Litscher, & Wassarman, 1998; Z. Williams et al., 2006). This evidence concerns the gene ZP3 and embryonal carcinoma.