ERBB2 and breast cancer: In fact, many tumor types are difficult to grow in immunocompromised mice (e.g. ER + and HER2 + breast cancer, though the high frequency of ER + breast cancers compensates for their poor PDX establishment rate, and alternative methods have improved engraftment rates [29]), or are rare in the population (e.g. angiosarcoma of the breast, phyllodes tumors) and are thus not readily available for engraftment.