PPP1R1B and breast cancer: To determine if these proteins have a direct effect on tumorigenesis and growth, we crossed Ppp1r1b knockout mice with the MMTV-PyMT mammary tumor model and examined the rate of tumor initiation, growth and metastasis in the wild-type (PyMT/Ppp1r1b+/+) and knockout (PyMT/Ppp1r1b-/-) Ppp1r1b backgrounds.