The D-type cyclins are known to be dispensable during mammary gland development, but are required for efficient tumor initiation as evidenced by the fact that mice lacking functional cyclin D1 are resistant to cancers initiated by ErbB-2/HER2/neu and ras oncogenes, while cyclin D3 null animals are refractory to Notch1-driven T cell acute lymphoblastic leukemia [7, 46, 47]. The gene discussed is ERBB2; the disease is T-cell acute lymphoblastic leukemia.