USP15 and breast cancer: Interleukin‐1 receptor type 2 (IL1R2) increases BMI1 deubiquitination and stability via binding and enhancing the activity of ubiquitin‐specific protease 15 (USP15) in cell nuclei, intrinsically promoting the self‐renewal of breast tumor initiating cells (BTICs) as well as breast cancer cell proliferation and invasion.