Zhang et al. developed an efficient method for the generation of somatic genetically-engineered mouse models for breast cancer through ex vivo expansion and genome editing of mammary stem cells; using this system, they showed that thee suppression of the tyrosine phosphatase PTPN22 promotes PIK3-driven tumorigenesis, inducing metastatic activity in these tumors [287]. Here, PIK3CG is linked to breast carcinoma.