 They form two-gene clusters (bigene clusters) with each other.There are DLX1-DLX2, DLX3-DLX4, DLX5-DLX6 clusters in vertebrates, linked to Hox gene clusters HOXD, HOXB, and HOXA respectively.In higher fishes like the zebrafish, there are two additional "DLX" genes, "dlx2b" ("dlx5") and "dlx4a" ("dlx8").