"BMP signals are mediated by type I and II BMP receptors and their downstream molecules Smad1, 5 and 8. Phosphorylated Smad1, 5 and 8 proteins form a complex with Smad4 and then are translocated into the nucleus where they interact with other transcription factors, such as Runx2 in osteoblasts." [http://www.ncbi.nlm.nih.gov/pubmed/15621726 (Chen 2004)].
Proteins on this pathway have targeted assays available via the [https://assays.cancer.gov/available_assays?wp_id=WP1425 CPTAC Assay Portal]