(a) MMTV-HER2 females (24-week-old) were distributed in groups and treated daily with the CDK4/6 inhibitor Abemaciclib (50 mpk), HC4 (50 mpk) or vehicle for 4 weeks. Abemaciclib treatment was followed by +/−HC4 (50 mpk) for 2 weeks. (b) IF of tumor sections for HER2, Ki67 (proliferation) and GADD34 (ER stress). Scale bars, 100 μm. (c) Macro-metastases (>100 cells) were detected by H&E staining and quantified in 5 lung sections/animal. P by Mann-Whitney test. (d) Micro-metastases (2-100 cells) were detected by IF staining using an anti-HER2 antibody and quantified per lung section/animal ±s.d. P by Mann-Whitney test. (e) Solitary DCCs were detected by IF staining for HER2, classified as Ki67+ or Ki67-and quantified per lung section±s.d. Vehicle N=18, HC4 N=17, Abema+ vehicle N+8, Abema +HC4 N=7. P by Mann-Whitney test.