Hh is necessary and sufficient for zic2a expression in the OS and ventral retina. (A-F) zic2a is expressed in the OS at 16s (A). In smo-/- mutants, zic2a expression is reduced in the OS at this stage (B, 14/14 embryos, n=2), but not elsewhere. shha mRNA-injected WT embryos express zic2a ectopically in the OS and retina at 16s (C, 13/13 embryos, n=1). zic2a expression is still reduced in smo-/- mutants at 19s (D,E, 30/30 embryos, n=2), and expanded in shha-injected retina (F, 19/19 embryos, n=1). The same result was observed at prim-5 (39/42 embryos, n=2, data not shown). Arrowheads point to the OS. Embryos are in lateral view, anterior to the left.