Zebrafish nmyc1 is prominently transcribed in tectal and retinal growth zones at early larval stages. Expression of nmyc1 on whole-mount embryos at 72 hpf, revealed by in situ hybridization with a digoxygenin-labelled nmyc1 antisense riboprobe (blue staining, left panels) or in combination with the M phase marker phospho-histone H3 (brown nuclei, right panels except I′) (all embryos anterior left, dorsal views in A,B, lateral views in C,D). (E–H) are frontal sections of the embryos in (C, D) at the levels indicated, (I–J) are parasagittal sections of similar embryos, with magnification of the caudal tectal border in (I′) and (J) (boxed area in I). Central nervous system expression of nmyc1 labels the circumference of the optic tectum (ot, black arrowheads), the retinal marginal zone (r, red arrowheads), the cerebellar plate (cb, green arrowheads) and rhombic lips in rhombomere 2 (rl, blue arrowheads). Note the striking coincidence between these sites and proliferating, PH3-positive cells.