Isochore (genetics)

A large region of DNA (greater than 3 KB) with a high degree uniformity in G-C and C-G (collectively GC) which tends to have more genes, higher local melting or denaturation temperatures, and different flexibility. Overall, Isochores are largely homogeneous in GC content in contrast to the heterogeneity of the entire genome.

There are five families of isochores ranging in overall GC content:
 * The L1 and L2 have a low GC-content of < 40% and have few genes
 * The H1 has a GC content of ~47%
 * The H2 has a GC content of ~52%
 * The H3 has the highest GC content of > 52% and has the highest density of genes.

Isochores have been described using CsCl ultracentrifugation.