ENPP2 and breast cancer: To analyze the function of endogenous expression of autotaxin by breast cancer cells in bone metastasis formation we used the RNA interference method to establish a series of three clones of 4T1 cells with stably down-regulated autotaxin expression (4T1-siATX), together with three control 4T1 clones with unaltered expression of autotaxin (4T1-sbATX) (Figure 4C).