The cytokine receptor family consists of a growing number of structurally and evolutionarily related transmembrane receptors. CRFB4 and IFNAR are two of the most similar members of this family. They are encoded by two neighboring genes on both human chromosome 21 and murine chromosome 16. The sequence of the human CRFB4 gene was determined from the first exon to the last intron. The nature of the repetitive sequences present in the introns was analyzed and compared with those present in the human IFNAR gene. This analysis leads to considerations of the antiquity of the duplication that gave rise to both genes from a common ancestor. A pseudogene for USF has been identified in the IFNAR gene and a new definition for the repetitive sequence MER37 is proposed. The polymorphism associated with two CA repeats present in the CRFB4 gene is described.