We investigated the genomic distribution of mouse and human repeated sequences 
by assessing their relative amounts in the four major components into which 
these genomes can be resolved by density gradient centrifugation techniques. 
These components are families of fragments that account for most or all of 
main-band DNAs, range in dG + dC content from 37% to 49%, and are derived by 
preparative breakage from long DNA segments (greater than 300 kb) of fairly 
homogeneous composition, the isochores. The results indicate that the short 
repeats of the B1 family of mouse and of the Alu I family of man are most 
frequent in the heavy components, whereas the long repeats of the BamHI family 
of mouse and of the Kpn I family of man are mainly present in the two light 
components. These results show that the genomic distribution of repeated 
sequences is nonuniform and conserved in two mammalian species. In addition, we 
observed that the base composition of two classes of repeats (60% dG + dC for 
short repeats; 39% dG + dC for long repeats) is correlated with the composition 
of the major components in which they are embedded. Finally, we obtained 
evidence that not only the short repeats but also the long repeats are 
transcribed, these transcripts having been found in mouse poly(A)+ mRNA.