Isoform a and isoform c differentially regulate mesendoderm formation and asymmetric organ development. Both isoforms regulate left-right development, but isoform a has a stronger effect on heart laterality. Isoform a and isoform c have distinct expression patterns during early mesendoderm development; isoform c is expressed symmetrically in presumptive mesendoderm during late blastula stages, and in both the anterior (polster) and posterior prechordal plate during late gastrulation. In contrast, isoform a expression begins at bud stage, in the anterior prechordal plate (polster). At 4- to 6-somite stages, isoform a and isoform c are both expressed in the polster, and weak isoform c expression remains in the posterior prechordal plate. Isoform a is also expressed in lateral head mesendoderm. Isoform a and isoform c are also expressed in different asymmetric domains during development of the diencephalon, gut and heart; isoform c is restricted to the dorsal diencephalon and left hand side of the developing gut, whereas isoform a shows left-sided asymmetric expression in the developing heart, with strongest expression in ther anterior regions. Isoform c is expressed during dome stage, while isoform a is not expressed until bud stage, following gastrulation. Isoform a shows strong expression during heart looping stages (30-36 hpf) and at 48 hpf. Isoform c is regulated by tgf-beta signaling in the early mesendoderm. Belongs to the paired homeobox family. Bicoid subfamily.