DKK1 and breast cancer: RT-PCR analysis of DKK1, DKK2, DKK3, and DKK4 in 4T1 murine mammary cancer cells showed that the expression levels of DKK1, 2, 3, and 4 increased in a dose-dependent manner, with the expression of DKK1 increasing more than 3-fold, the expression of DKK2 and 3 increasing about 2-fold, and DKK4 expression levels showing a nonsignificant increase [85].