A map of the type-I interferon gene cluster located on the short arm of human chromosome 9 (9p) has been constructed using a contig of YAC clones. This map contains 26 interferon (IFN) genes and pseudogenes, and it accounts for all, except one, of the IFN sequences previously reported by other authors, plus a new IFNW pseudogene. The most distal gene on 9p is IFNB, and the most proximal one is IFNWP19. The direction of transcription for the 20 most distal IFN sequences is toward the telomere and for the 6 most proximal sequences, toward the centromere. Several regions of the cluster show evidence of ancestral duplication events. Some of these events may be explained by unequal crossing over between adjacent tandem genes. The location of several breakpoints within the cluster, from deletions associated with leukemias and gliomas, was also determined.