MCTS1 (malignant T cell amplified sequence 1), also known as MCT1, is a 181 amino acid protein that is ubiquitously expressed and localizes to the cytoplasm of cells. MCTS1 may play a role in cell cycle regulation by decreasing cell doubling time and by shortening the duration of G1 transit time and G1/S transition. The protein level of Cdk4 and Cdk6 kinases activity and cyclin D1 is enhanced by expression of MCTS1. As a translation enhancer, MCTS1 recruits DENR and binds to the cap complex of the 5'-terminus of mRNAs, subsequently altering the mRNA translation profile. MCTS1 promotes lymphoid tumor development and contributes to the pathogenesis and progression of breast cancer. MCTS1 positively regulates phosphorylation of ERK1 and ERK2.