KCNN4 and breast neoplasm: In order to validate the relevance of SK4 for mammary tumour development in vivo, we crossed the well‐established polyoma middle T‐antigen (PyMT) and epidermal growth factor receptor 2 (cNeu) oncogene‐driven mouse breast tumour models to gene‐targeted mice lacking functional SK4 (Guy et al., 1992a,b; Sausbier et al., 2006).