Fig. 5. A proposed mechanism for the regulation of events leading to sperm capacitation. Ejaculated sperm entering the female reproductive tract are exposed to a FGFR-1 ligand. The ligand induces a FGFR-1 signaling cascade. Activation of the PI3K pathway results in the phosphorylation of a `master controller of capacitation' (MCC), which suppresses downstream tyrosine phosphorylation of signal transduction pathways including protein kinase A (Konopka et al., 1984) activation. Following the disengagement of FGFR-1 from its ligand, or the suppression of the PI3K pathway, PTPs dephosphorylate the MCC, allowing the tyrosine phosphorylation and activation of target proteins leading to the correlates of sperm capacitation. Furthermore, activation of the PI3K pathway results in the suppression of the MAPK pathway. The level at which this suppression occurs is not known, however, it is upstream of ERK 1/2.