UBB and neoplasm: Indeed, we found that overexpression of wild-type ubiquitin in different human cell lines (both normal and tumor derived) resulted in lowered levels of UBC and UBB mRNAs; moreover, the UBC fold-decrease was directly related to the amount of ubiquitin overexpressed, suggesting that a proper negative feedback regulatory mechanism, able to sense the Ub levels, could act to maintain Ub within a defined concentration range under unstressed conditions.