Potent and highly selective inhibitor of BMP signaling, with low nanomolar IC50 values of 1.8, 1.1, 6.4 nM for ALK1, ALK2 and ALK6 respectively and IC50s of 34.4, 302, 321 and 220 nM for other ALKs (3, 4, 5) and ActRIIA respectively.K02288 specifically inhibited the BMP-induced Smad pathway without affecting TGF-β signaling and induced dorsalization of zebrafish embryos. K02288 provides a useful tool to investigate BMP signaling and to research into stem cell biology and disease models of anemia, musculoskeletal dysplasia and cancer.