<prompt> Primitive streak is initiated and maintained by?Nodal geneBMP4FGFBrachyury gene  <response>: The primitive streak itself is initiated and maintained by expression of Nodal, a member of the transforming growth factor-b TGF-b family. Once the streak is formed, Nodal upregulates a number of genes responsible for formation of dorsal and ventral mesoderm and head and tail structures. Another member of the TGF-b family, bone morphogenetic protein 4 BMP4, is secreted throughout the embryonic disc. Ref: Langmans Medical embryology 10e, p 52-53. 