Expression of hoxda cluster genes during pectoral fin development in zebrafish. Expression of hoxda genes occurs in three phases during pectoral fin bud development. Phase I, which occurs at the beginning of fin bud morphogenesis, is characterized by a uniform expression of hoxd9a within the early fin bud mesenchyme (A, F, K, P, U). Phase II, which immediately follows phase I (and for hoxd10a (F), occurs simultaneously with phase I), is characterized by a sequential activation of hoxd10–13a gene expression, each of which occupies successively smaller regions within the fin bud centered at the posterior margin (“early” phase II: B, G, L, Q, V). During later stages (“late” phase II) distal expression domains of these genes extend variably into more anterior regions, thereby causing an appearance of the distal bending of the expression (C, H, M, R, W). Phase III, which is limited to the distal fin bud mesenchyme cells, occurs last and is characterized by the posteriorly limited expression of hoxd11–13a genes within the most distal group of cells (O, T, Y). Note that, unlike Hoxd13 in tetrapod limbs, expression of hoxd13a in zebrafish pectoral fins during this phase does not cover the entire distal fin bud region. During phase III, no expression is seen for hoxd9a and hoxd10a in distal cells (E, J). Lateral (A–D, F–I, K–N, P–S, U–X) or dorsal (E, J, O, T, Y) views with anterior to the left in all panels. Only left pectoral fin buds are shown. Black and white arrows in panels O, T, Y show the anterior limits of expression within the proximal and distal mesenchyme cells, respectively. hpf: hours post fertilization.