Post-synthetic modifications (e.g., phosphorylation, acetylation, methylation) of the chromosomal proteins have been the subject of several reviews. Acetylation of histones in particular, and phosphorylation of the nonhistones, have been correlated to transcriptional activation of chromatin, possibly by relaxation of constraints on the associated DNA sequences. The High mobility group (HMG) class of the nuclear nonhistone proteins is among the most studied. The HMG proteins are so defined because of their relative mobilities in acid-urea polyacrylamide gels. Arfmann et al. have reported phosphorylation of all four HMG proteins in interphase and metaphase Chinese hamster ovary cells. They observed that HMG-17 was the predominant phosphate acceptor during interphase. The cyclic nucleotide-independent kinases NI and NII, and the cyclic nucleotide-de-pendent kinases, have been shown to phosphorylate HMG proteins in vitro. HMG-2 is known to exist in multiple electrophoretic forms. The microheterogeneity of HMGs and the tissue-specific variations in their distribution may, at least in part, be a reflection of their post-synthetic modifications.