Accumulating evidence has documented CCNA2 is an important differentially expressed gene (DEG, mainly overexpressed) in various cancer types compared to normal tissues, such as LIHC (Li et al., 2021a), human epidermal growth factor 2 (HER2)+ breast cancer (Weng et al., 2021), LUSC (Gao et al., 2020), COAD (Li et al., 2021b), PRAD (Feng et al., 2021), HNSC (Zhang et al., 2020), KIRC (Zhan et al., 2021), THCA (Li et al., 2020b), medulloblastoma (Guo et al., 2020a), gastric cancer (Ji et al., 2021; Lu et al., 2021), and mantle cell lymphoma (Guo et al., 2020b). Here, ERBB2 is linked to breast cancer.