CCND1 and breast cancer: Several studies have shown that cell lines with elevated CCND1 and decreased CDKN2A (p16) expression are the most sensitive to CDK4/6 inhibitors.34 PD‐0332991, a selective CDK4/6 inhibitor, is being tested in advanced breast cancer with CCND1 amplification and/or CDKN2A (p16) loss.