A segment of DNA that has a uniform base composition that is different from adjacent segments. The DNA of vertebrates and plants are mosaics of such isochores. In humans, isochores are about 300 kb in length and consist of five classes. The AT-rich isochores are called L1 and L2, and the GC-rich isochores are H1, H2, and H3. Although H3 makes up only 3% of the total DNA, it contains over 25% of the ORFs. See major histocompatibility complex (MHC).
Subjects: Genetics and Genomics — Medicine and Health.