MDM2 and neoplasm: In addition, increased expression of the tumour suppressor Cdkn2a (p19Arf) was also detected, which may have been involved in stabilization of the p53 tumour suppressor by inhibition of Mdm2, or in promoting cell cycle arrest prior to apoptosis together with up-regulation of the p53-target gene Cdkn1a (p21Cip1).