Chromatin isolation for histone extracts like H2A, H2B, H3, and H4 extraction for SDS-PAGE electrophoresis. Histones act as spools around which DNA winds, and also play a role in gene regulation. The core histones include H2A, H2B, H3, and H4. Histones undergo post-translational modifications, which alter their interaction with DNA and nuclear proteins. The H3 and H4 histones have long tails protruding from the nucleosome, which can be covalently modified at several places (i.e., methylation, acetylation, phosphorylation, etc.). Histone modifications act in diverse biological processes such as gene regulation, DNA repair and chromosome condensation (mitosis). Combinations of modifications are thought to constitute a "histone code" and an understanding of this code may help to answer questions about human development and disease.