ID1 and breast carcinoma: In a way, these transfected cell lines are somewhat reminiscent to the few breast cancer cell lines that express GREM1. In these GREM1-overexpressing cell lines, BMP-induced SMAD1/5/8 phosphorylation (Fig. 4a) and expression of BMP target genes ID1 and 3 (Fig. 4b) were clearly inhibited.