SMD4,also named as MCB1,AF or ASF,belongs to the proteasome subunit S5A family. It binds and presumably selects ubiquitin-conjugates for destruction. PSMD4 displays selectivity for longer polyubiquitin chains. It modulates intestinal fluid secretion. PSMD4 is exposed on the sperm acrosomal surface,a notion that was further supported by the binding of non-cell permeant,biotinylated proteasomal inhibitor ZL3VS to the sperm acrosome. It is involved in the sperm-ZP penetration during fertilization. This antibody is specific to PSMD4.