The most deviant isochore pattern within mammals was found in rat and mouse; 
most other mammals possess a different kind of isochore organization called the 
"general pattern." However, isochore patterns remain largely unknown in rodents 
other than mouse and rat. To investigate the taxonomic distribution of isochore 
patterns in rodents, we sequenced the nuclear gene LCAT (lecithin:cholesterol 
acyltransferase) from 17 rodents species (bringing the total of LCAT sequences 
in rodent to 19) and compared their GC contents at third codon positions and in 
introns. We also analyzed an extensive sequence database from rodents other than 
rat and mouse. All murid LCAT sequences are much poorer in GC than all nonrodent 
LCAT sequences, and the hamster sequence database shows exactly the same 
isochore pattern as rat and mouse. Thus, all murids share the same special 
isochore pattern--GC homogenization. LCAT sequences are GC-poor in 
hystricomorphs too, but the guinea pig sequence database indicates that large 
changes in GC content occur without an overall modification of the isochore 
pattern. This novel mode of isochore evolution is called GC reordering. LCAT 
sequences also show that the evolution of isochores in sciurids and glirids is 
nonconservative in comparison with that in nonrodents. Thus, at least two novel 
patterns of isochore evolution were found. No rodent investigated to date shared 
the general mammalian pattern.