Quantitative PCR analyses showed that the levels of MdmX messenger RNA (mRNA) in transfected cells (FL, 14.2±5.7, dRING, 11.0±5.8; dZnF, 13.3±3.6; dZnF-dRING, 35.2±4.5) are similar to the endogenous MdmX levels in p53 null mouse tumor cells (13.7±4.5) and in wild-type MEFs (11.07±4.6). This evidence concerns the gene MDM4 and neoplasm.