1,25D markedly suppressed HAS2 and reduced HA secretion by breast cancer cells, and the growth inhibitory effects of 1,25D3 were partially reversed in the presence of exogenous HA.(22) HAS2 expression and HA production are elevated in models of mammary EMT, indicating that deregulation of HA production occurs early in breast tumorigenesis and is associated with the emergence of CD44+ stem cells. The gene discussed is HAS2; the disease is breast cancer.