Effects of PD 0332991, tamoxifen, and trastuzumab on growth of breast cancer cell lines. PD 0332991 (PD) acts synergistically with tamoxifen (Tam) and trastuzumab in inhibiting growth of (a) estrogen receptor-positive human breast cancer cell lines and (b) HER2-amplified cell lines in vitro, respectively. (a) Grey squares, PD 0332991 alone; open diamonds, tamoxifen alone; dark triangles, PD 0332991 and tamoxifen combination. (b) Grey squares, PD 0332991 alone; open diamonds, trastuzumab alone; dark triangles, PD 0332991 and trastuzumab combination. Error bars represent the standard error for two separate experiments. Mean combination index (CIm) for the combination curves shown with standard error CIm < 1, indicating synergy for the combinations.