PIN1 and breast cancer: Using concentrations (0.1–2 μM) that have widely and safely been used in APL cells47–49, we surprisingly found that ATO dose-dependently downregulated Pin1 protein levels in mouse embryonic fibroblasts (MEFs) (Fig. 1a, b), human TNBC MDA-MB-231 (231) cells (Fig. 1c, d) and many other breast cancer cells (see below).