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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. (PMID: 20526895) This antibody is specific to PSMD4.