Furthermore, the isomerase activity of CypA is essential for tumorigenesis and the proliferation of estrogen receptor (ER)+ and ER− breast cancers through Janus kinase 2 (JAK2)/signal transducers and activators of transcription 5 (STAT5) activation, implying that PPIase activity is essential for cancer cell proliferation [65]. This evidence concerns the gene JAK2 and cancer.