An archetypal example is the switch of alternative 5′ splice site in exon 2 of BCL2L1, generating functionally distinct RNA and protein isoforms BCL-xL and BCL-xS that are anti- and pro-apoptotic, respectively, with the former overexpressed in many types of cancers to promote tumor progression, including breast cancer, non-small cell lung cancer and prostatic cancer (Boise et al., 1993; Castilla et al., 2006; Espana et al., 2004; Ikegaki et al., 1994; Trisciuoglio et al., 2017). The gene discussed is BCL2L1; the disease is breast cancer.