| Consistent with these results , the endogenous ICER protein is elevated in cells which are null for murine Rad6B ( mHR6B / ) or transfected with dominant negative and antisense constructs of human CDC 34 . ^^^ Based on the requirement for CREM / ICER and Rad6B proteins in spermatogenesis , we determined expression of Cdc 34 , Rad6B , CREM / ICER isoforms , and the Skp 1 Cullin F box ubiquitin protein ligase subunits Cul 1 and Cul 2 , which are associated with Cdc 34 activity during murine testicular development . ^^^ |