Looking for relationships between the most repeated dispersed DNA sequences in the mouse: small R elements are found associated consistently with long MIF repeats.

Four highly-repeated dispersed DNA sequence families have been described in the mouse genome. These are the three small elements B1, B2 and R and the large (6 kb) MIF element. Together these comprise approximately 10% of the mouse genome. Possible relationships between these families are pertinent to the genome as a whole. We report here that the B1s, B2s and Rs are all randomly organized in the genome with respect to each other. Surprisingly though, the R and MIF families are found together consistently in a set of random genomic clones and in selected clones. We find Rs often located on one end of the MIF at a consistent site and conclude that a minority of Rs are an integral part of MIF while the majority of Rs are not associated with MIFs. We propose that isolated R elements are truncated forms of MIF. Also we speculate on the mechanism of dispersal of these elements through the mouse genome.