We found that decreased expression of circ_6014 decreased the protein expression levels of PCNA, CDK2/CCNE1, and CDK4/6/CCND1, and overexpression of circ_6014 increased the expression of these proteins in breast cancer cells (Figure 2K–2N) (Student’s t test, p < 0.05). This evidence concerns the gene CCND1 and breast cancer.