A pooled DNA probe from P1 artificial chromosome clones (PACs) containing the human lecithin:cholesterol acyl transferase (LCAT) gene cluster was used in fluorescence in situ hybridization (FISH) experiments assigning the genes to pig chromosome 6p13. In addition, probes derived from the coding regions in the human gene cluster were used in long range mapping experiments to show that the overall structures of the human and porcine LCAT gene clusters are identical. Both the linear order and the close physical distance of five apparently unrelated genes have been maintained throughout 90 million years of divergent evolution between human and pig. The extremely dense clustering of the genes in the LCAT gene cluster suggests that this gene organization has biological significance. The conservation of the gene cluster between human and pig supports this suggestion.