These results together with our findings that PEAK1 expression predicts low median overall survival in breast cancer patients with high INHBA transcript levels and enriched for MSC content (Supplementary Fig. 8c), further support a role for PEAK1-dependent INHBA/activin-A expression as a mechanism by which stromal MSCs support HER2-positive breast cancer progression and therapy resistance. Here, ERBB2 is linked to breast carcinoma.