A cell-permeable rhodanine compound that upregulates cellular p21Cip1/Waf1 and p27Kip1 protein levels (10μM for 16 h in 501 Mel cultures) by preventing Skp2-Cks1-mediated substrate recruitment without affecting the F-box E3 ubiquitin ligase Skp2 interaction with Cks1 or the SCF (Skp1-cullin1-F-box) c