In genetics, an isochore is a large region of DNA (greater than 300 KB) with a high degree uniformity in guanine (G) and cytosine (C): G- C and C-G (collectively GC content).
Bernardi and colleagues first uncovered the compositional non-uniformity within 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 isochore’s composition varied markedly between "warm-blooded" (homeotherm) vertebrates and "cold-blooded" (poikilotherm) vertebrates and later became known as the isochore theory.
Read more about Isochore (genetics): The Thermodynamic Stability Hypothesis, Principles of The Isochore Theory, The Neutralist-Selectionist Controversy, The Rise and Fall of The Isochore Theory, The Compositional Domain Model