The absent small and homeotic (ash2) gene is a member of the trithorax group of positive transcriptional regulators of the homeotic genes. In this work we present evidence that loss-of-function of ash2 results in patterning alterations in the developing wing. Homozygous adults of the weak allele ash2(112411) develop extra cross-veins. However, clonal analysis of a stronger allele, ash2(11), shows that this allele results in reduction of intervein tissue and increase of longitudinal veins and cross-vein tissue in the wing except the region between vein L3 and L4. These results suggest that ash2 function is required for both activation of intervein tissue and repression of vein tissue. Moreover, we have found that cross-vein development can be rescued in the absence of crossveinless-2 when the levels of ash2 were reduced.