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A 45‐kb DNA domain with two divergently orientated genes is the unit of organisation of the murine major urinary protein genes.

The multigene family which codes for the mouse major urinary proteins (MUPs) consists of approximately 35 genes. Most of these are members of two different groups, Group 1 and Group 2, which can be distinguished by nucleic acid hybridisation. By screening a Charon 4A library of mouse DNA with probes from the 5′‐flanking region of a MUP gene, we have isolated clones that contain both a Group 1 and a Group 2 gene, orientated in a divergent fashion, with 15 kb of DNA between the 5′ ends of the genes. We show that this pairwise arrangement is the predominant organisation of MUP genes in the BALB/c genome. We argue that the head‐to‐head gene pair is the unit both of DNA organisation and of evolution. Taking into account the genes themselves, the intervening 15 kb and the homologous 3′‐flanking regions, this unit is approximately 45 kb long. We also show that some MUP genes may be linked in a tail‐to‐tail fashion with 26‐28 kb between the 3′ ends of two genes. This suggests that the minimum distance between successive 45‐kb units is approximately 7 kb.