Five related cytokine genes, interleukin 3 (Il3), interleukin 4 (Il4), interleukin 5 (Il5), interleukin 13 (Il13), and granulocyte-macrophage colony-stimulating factor (Csfgm or Csf2), are tightly linked on mouse chromosome 11. We now describe a 1-Mb transcript map of this cytokine cluster. Genomic clones obtained by screening mouse bacterial artificial chromosome (BAC) and P1-derived artificial chromosome (PAC) libraries were subcloned into the pSPL3 expression vector and transfected into COS7 cells for exon trapping. In total, 118 distinct, putative exons were sequenced and characterized, mapping up to 29 distinct genes to the mouse cluster, including Il4 and Csf2. Northern blot and RT-PCR analyses indicate that all of these genes are expressed. Analysis of 1 Mb of published sequence from the region of conserved synteny on human chromosome 5q31-q33 identified 45 gene candidates, including 35 expressed genes in the human IL-4 cytokine gene cluster. Probes for 20 human genes were tested for cross-hybridization to murine BAC and PAC clones, thereby mapping 11 additional genes to the mouse complex. Thus, a total of 40 genes including 6 cytokine genes have been physically mapped within 1 Mb of mouse chromosome 11. Gene order in this complex is similar, but not identical, between human and mouse. The integrated physical and transcript maps should prove valuable as a complement to genomic sequencing and expression-dependent transcript maps of this segment of the genome.