To identify novel oncogenic E3 ubiquitin ligases as anti-cancer targets, we screened an E3 ubiquitin ligase siRNA library containing siRNA pools against 555 individual E3s using the SRB assay in the MDA-MB-231 breast cancer cell line and the PC3 prostate cancer cell line.RNF126 was identified and validated as a candidate from this screening.Knockdown of RNF126 dramatically decreased cell viability in these cancer cell lines.Consistently, RNF 126 knockdown delayed cell cycle G1/S progression and decreased cell proliferation.Using protein array analysis we found that RNF126 silencing increased cell cycle dependent kinase inhibitor p21cip protein levels in both MDA-MB-231 and PC3.Knockdown of RNF126 stabilized the p21 protein rather than increase p21 mRNA levels.We demonstrated that RNF126 interacts with p21 and RNF126 over-expression increased p21 protein ubiquitination in an E3 ligase activity-dependent manner.RNF126 knockdown induced loss of cell viability in MDA-MB-231;PC-3 can be partially rescued by depletion of p21.RNF126 stable knockdown in SW527 inhibited tumor growth in SCID mice.Finally,we found that RNF 126 is highly expressed in a subset of breast cancer cell lines and negatively correlated with p21 expression levels.These findings suggest that RNF 126 promotes cancer cell proliferation by targeting p21 for ubiquitin-mediated degradation.RNF126 could be a novel therapeutic target in breast and prostate cancers.The research is supported by grants from National Nature Science Foundation of China (U1132605, 81072162, and 81120108019),and a grant from Yunnan Province High-Profile Scientist Program (2010CI114).