PSMD4 and oculopharyngeal muscular dystrophy: Measurements of the chymotrypsin-like activity in OPMD muscles in the presence of heterozygous mutants of proteasome subunits showed that it was reduced to wild-type levels by the Rpn10 mutation, although this reduction was not observed with mutants of the other proteasome subunits, Rpn11 and Prosβ4 (Fig 4A).