Cerberus protein on the left side of chick embryos activates:
At stage when left-right asymmetry established, Sonic hedgehog expressed left of Hensen's node induces Cerberus-related Caronte in left paraxial and lateral plate mesoderm. Caronte functions as high-affinity BMP antagonist binding BMP2, BMP4, BMP7 extracellularly, preventing Smad1/5/8 phosphorylation that normally represses Nodal promoter via interplay with FoxH1 and Smad2. BMP inhibition lifts repression permitting Nodal autoactivation left side, followed by Lefty and Pitx2 expression directing asymmetric organ morphogenesis. Right side lacking Caronte maintains BMP blocking Nodal, explaining indirect Nodal activation by Cerberus.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 12: Cerberus activates Nodal via BMP inhibition in left-right patterning.