Fourth, goosecoid is a primary response gene induced by activin. Finally, goosecoid mRNA induces secondary axes, in which the expression of goosecoid protein in the ventral side of the embryo is capable of starting a new body axis. The goosecoid mRNA can also form complete head structures, with tail development, and notochord tissue, which are characteristics of the Spemann organizer. DeRobertis, Eddy M., Guillermo Oliver, and Christopher V.E. Wright. Homeobox Genes and the Vertebrate Body Plan.
Homeobox genes are genes that regulate the activity of other genes.
All vertebrates have four homeobox complexes, each on separate chromosomes. Genes expressed posteriorly are located near the left end of the complex while those expressed anteriorly are at the right end. Therefore, the genes of the homeobox are arranged in chromosomal DNA in the same order in which they are expressed along the anterior-posterior body axis. To understand how these genes allow for cellular differentiation during development, scientists have studied the proteins produced by the homeobox genes and their location in the embryo at several developmental stages. One such gene and its protein in the Xenopus XIHbox 1 protein.