Fig. 7. A model for the establishment of Krox20 expression in r3. (A-C) The model shows the evolution of the expression domains of Krox20 and of different established (Pbx, Meis2, Hoxa1/b1) or postulated (X) regulators at different stages of development (A, E7.5; B, E8; C, E8.5). The overlap between the Hoxa1/Hoxb1 and X domains results in a narrow strip of cells activating Krox20 and transiently co-expressing PG 1 genes. These cells then shut down PG 1 gene expression, but maintain Krox20 expression by direct and indirect autoregulatory loops, and r3 extends rostrally by the recruitment of neighbouring cells by non cell-autonomous autoregulation. As the X domain extends posteriorly, the strip of co-expression of Krox20 and PG 1 genes moves caudally, leading to the progressive extension of r3 at the expense of r4. In the rest of r4, Krox20 is repressed by mechanisms downstream of Hoxa1/Hoxb1. See Discussion for further details. The potential role of Hox PG 2 factors in initial activation is not represented. Expression domain extensions are indicated by coloured arrows. Positive and negative regulations are symbolized by arrows and barred lines, respectively. Direct and indirect regulations are indicated by plain or dotted lines, respectively. r, rhombomere; pr, prospective rhombomere.