Human sperm binding to the zona pellucida requires human ZP2. (A) Ovarian histology from huQuad^(huZP1–4) and huQuad-ZP2^Null mice as in Fig. 1 C. (B) Eggs from huQuad-ZP2^Null stained with monoclonal antibodies as in Fig. 1 D. (C) Human sperm binding to huQuad^(huZP1–4), huQuad-ZP2^Null, and huQuad-ZP2^Null; Zp2^Mo eggs (as in Fig. 1 E) using noninseminated human oocytes and mouse Zp3^EGFP eggs (green zona) as positive and negative controls, respectively. (D) Litter sizes after transcervical insemination of control (Cd9^+/−) mice compared with natural mating (top). Sperm in the perivitelline space (PVS) of Cd9^−/− eggs after transcervical insemination with mouse sperm (bottom). Recovered eggs (left) and the number of sperm in PVS (right). Arrows indicate sperm. (E) As in D (bottom) but with huZP2^Rescue (top) and huZP3^Rescue (bottom) eggs after transcervical insemination with human sperm. (F) In vivo oviduct transfer of human sperm (2.3 × 10³ sperm in 0.5 µl) to hormonally stimulated, anesthetized huZP2^Rescue and huZP3^Rescue female mice. (G) As in E, but after in vivo oviductal transfer. (H) Normal mouse, huZP2^Rescue, huQuad eggs, and human oocyte stained with antibody to the sialyl-Lewis^X antigen.