We are studying murine intracisternal A-particles as a model for the evolution and expression of integrated reiterated viral genes. Multiple individual A-particle genes have been isolated from mouse DNA using recombinant DNA techniques. These genes number 1000-2000 copies per cell in Mus musculus and closely related species, but only 50 copies are found in the DNA of more distant members of this genus. Rat and Syrian hamster DNA contain about 700 and 1600 copies per cell of IAP-related genetic units, respectively; and individual genes have been isolated from gene libraries of both these animals. The rat gene family shows a much higher degree of internal divergence than do those of the mouse and Syrian hamster. Heteroduplex analysis and melting data suggest that the IAP gene family has undergone independent amplification events at different times in the evolution of these rodent species.