PCNA and breast cancer: Next, the results of CCK‐8 and Western blot assay showed that the viability of breast cancer cells transfected with either si‐hsa_circRNA_002178‐1 or si‐hsa_circRNA_002178‐2 was inhibited, accompanied with reductions in expression of proliferation‐related proteins PCNA and Ki67 (P < .05) (Figure 3B,C).