In genetics, an isochore is a large region of genomic DNA (greater than 300 kilobases) with a high degree of uniformity in GC content; that is, guanine (G) and cytosine (C) bases. The distribution of bases within a genome is non-random: different regions of the genome have different amounts of G-C base pairs, such that regions can be classified and identified by the proportion of G-C base pairs they contain.
Bernardi and colleagues first noticed the compositional non-uniformity of vertebrate genomes using thermal melting and density gradient centrifugation.
 The DNA fragments extracted by the gradient centrifugation were later termed "isochores", which was subsequently defined as "very long (much greater than 200 KB) DNA segments" that "are fairly homogeneous in base composition and belong to a small number of major classes distinguished by differences in guanine-cytosine (GC) content". Subsequently, the isochores "grew" and were claimed to be ">300 kb in size."
 The theory proposed that the isochore composition of genomes varies markedly between "warm-blooded" (homeotherm) vertebrates and "cold-blooded" (poikilotherm) vertebrates and later became known as the isochore theory.