Antagonist of signaling by TGF-beta (transforming growth factor) type 1 receptor superfamily members; has been shown to inhibit selectively BMP (bone morphogenetic proteins) signaling by competing with the co-SMAD SMAD4 for receptor-activated SMAD1. SMAD6 is an inhibitory SMAD (I-SMAD) or antagonistic SMAD. Binds to regulatory elements in target promoter regions. Interacts with NEDD4L. Interacts with WWP1. Interacts with RNF111 and AXIN1. Interacts with TGF-beta type I receptor superfamily members, SMAD1, HOXC8 and HOXC9. Interacts with STAMBP and PRKX. Ubiquitous in various organs, with higher levels in lung. Isoform B is up-regulated in diseased heart tissue. Belongs to the dwarfin/SMAD family. 3 isoforms of the human protein are produced by alternative splicing. Note: This description may include information from UniProtKB.