The zona pellucida (ZP) is a transparent envelope that surrounds the mam-malian oocyte and mediates species-selective sperm–egg interactions. Por-cine and bovine ZPs are composed of the glycoproteins ZP2, ZP3, and ZP4. We previously established an expression system for porcine ZP glyco-proteins (ZPGs) using baculovirus in insect Sf9 cells.