CPSF4 and breast cancer: To more directly assess the role of NANOS1, PUM2, and CPSF4 in metastasis, we knocked down (KD) their expression in two metastatic mouse mammary cancer cell lines, 4T1 and 6DT1, using short hairpin RNAs (shRNAs) (Fig. S1a–b) and performed spontaneous metastasis assays by mammary fat pad injection into syngeneic (BALB/c for 4T1, FVB/N for 6DT1) mice.