The bone morphogenetic protein (BMP) signalling pathway play critical roles in germ layer induction and patterning in early zebrafish embryos. Smad2/3 and Smad1/5/8 are intracellular effectors of BMPs. These Smads regulate, in cooperation with other factors, transcription of hundreds of target genes in the nucleus. The activity and stability of Smads are regulated by phosphorylation modifications.