Sphox11/13b is one of the two hox genes of Strongylocentrotus purpuratus expressed in the embryo. Its
dynamic pattern of expression begins during gastrulation, when the transcripts are transiently located in a
ring of cells at the edge of the blastopore. After gastrulation, expression is restricted to the anus–hindgut
region at the boundary between the ectoderm and the endoderm. The phenotype that results when translation
of Sphox11/13b mRNA is knocked down by treatment with morpholino antisense oligonucleotides (MASO)
suggests that this gene may be indirectly involved in cell adhesion functions as well as in the proper
differentiation of the midgut–hindgut and midgut–foregut sphincters. The MASO experiments also reveal that
Sphox11/13b negatively regulates several downstream endomesoderm genes. For some of these genes,
Sphox11/13b function is required to restrict expression to the midgut by preventing ectopic expression in the
hindgut. The evolutionary conservation of these functions indicates the general roles of posterior Hox genes
in regulating cell-adhesion, as well as in spatial control of gene regulatory network subcircuits in the
regionalizing gut
