Cyclin D3 and cyclin-dependent kinases 6(CDK6) are components of the core cell cycle machinery driving cell proliferation, and generally amplified in T acute lymphoblastic leukemia (T-ALL). The pro-survival function of cyclin D3/CDK6 in T-ALL cells via phosphorylating/inactivating PFKP and PKM2 (two key enzymes in glycolysis) is recently addressed. Targeting cyclin D3/CDK6 is a promising method for T-ALL therapy. As tumor microenvironment cell play critical roles in regulating tumor progression, it is critical/urgent to understand the functions of cyclin D3/CDK6 in tumor microenvironment cells. Our preliminary study showed that cyclin D3/CDK6 expression was significantly upregulated in T cells under activation condition. Ablation of cyclin D3 or CFK6 dramatically decreased regulatory T cell (Tregs) population without inducing cell apoptosis. Tregs promote tumor progression by inhibiting T help cells and CD8+ cytotoxic T cells. We therefore hypothesized that cyclin D3/CDK6 regulate T-ALL progression via T-ALL prosurvival and Tregs differentiation. This study will provide the rational to treat T-ALL by targeting cyclin D3/CDK6.