There is a gradual increase in the double phosphorylation of the Thr-Glu-Tyr motif, characteristic of activated kinases such as ERK1, 2 and 5 in two proteins of 80 and 105 kDa in capacitating human spermatozoa (de Lamirande and Gagnon, 2002; Thundathil et al., 2002, 2003).