In this study, we showed that Roquin1 is a potent breast tumor suppressor that induces tumor cell cycle arrest by selectively suppressing the expression of cell cycle–promoting genes, including CCND1, CCNE1, CDK6, and MCM2. Roquin1 expression was reduced in breast cancer tissues and cells, which might contribute to their lack of cell cycle regulation. The gene discussed is MCM2; the disease is breast carcinoma.