tokens: Based on the requirement for CREM / ICER and Rad6B proteins in spermatogenesis , we determined expression of Cdc34 , Rad6B , CREM / ICER isoforms , and the Skp1 - Cullin - F - box ubiquitin protein ligase subunits Cul - 1 and Cul - 2 , which are associated with Cdc34 activity during murine testicular development .