HOXB1 and carcinoma: The fact that hoxb1a expression persists at high levels until at least 24 hpf in zebrafish, and the lack of ectopic hoxb1a expression in the zbtb16a−/−;zbtb16b−/− mutants, suggests that Zbtb16 is not regulating fgf3 through hoxb1. GATA2 and GATA3 specify r4 identity downstream of Hoxb1 in the mouse hindbrain (Pata et al., 1999), and GATA4 mediates fgf3 upregulation upon retinoic acid (RA)-induced differentiation of F9 embryonal carcinoma cells into parietal endoderm (Murakami et al., 1999), a cell type which also expresses hoxb1 (Boylan et al., 1993).