Nature 407 , 527 - 530 (2000 ).

This paper contained a mistake in Fig. 4a–d. The correct data are described below and are shown on our web site (http://www.med.upenn.edu/~cellbio/arrow.html). The main conclusions remain unchanged. In the paper, we used a LacZ enhancer trap inserted at Distal-less to investigate its regulation by arrow. However, as both Distal-less and arrow are on chromosome 2R, the LacZ reporter was flipped away in making arrow mutant clones. We have repeated these experiments using anti-Distal-less antibodies generating marked, Minute+ arrow 2 mutant clones. Early-induced arrow mutant clones do not survive well; those that do survive exhibit loss of Distal-less expression. Later-induced clones show reduced Distal-less, although there is still residual expression. It is possible that the residual expression is due to perdurance of Arrow protein in cells only recently made mutant for arrow. As we stated in the paper, these results with arrow are quite similar to those obtained for clones mutant for frizzled frizzled2 (C. M. Chen and G. Struhl, Development 126, 5441–5452; 1999). In the legend to Fig. 4, however, we noted the higher level of Distal-less expression in the wild-type twin spots compared to the heterozygous tissue. Rather than reflecting changes in Wnt signalling, this resulted simply from the presence of two copies of the Distal-less LacZ reporter on the wild-type twin spots.