The distribution of the G+C content in the mouse genome has been studied using a 
windowless technique. We have found that: (i). Abrupt variations of the G+C 
content from a GC-rich region to a GC-poor region, and vice versa, occur 
frequently at some sites along the sequence of the mouse genome. (ii). Long 
domains with relatively homogeneous G+C content (isochores) exist, which usually 
have sharp boundaries. Consequently, 28 isochores longer than 1 Mb have been 
identified in the mouse genome. A homogeneity index was used to quantify the 
variations of the G+C content within isochores. The precise boundaries, sizes, 
and G+C contents of these isochores have been determined. The windowless 
technique for the G+C content computation was also used to analyze the DNA 
sequence containing the mouse MHC region, which has a GC-poor isochore. This 
isochore is located at the central part of the sequence with boundaries at 
468459 and 812716 bp, where the sequence is extended from the centromeric end to 
the telomeric end. In addition, the analysis of a segment of the rat genome 
shows that the rat genome also has clear isochore structures.