We have identified a number of nonallelic histone variants which are expressed differentially during development and differentiation of mammalian tissues. Some of these histone variants are evidently involved in the maintenance and repair of chromatin structure, an important mechanism for preserving the normal pattern of gene expression of long-lived nondividing cells. We are continuing to investigate the metabolism of nucleosome proteins in murine erythroleukemia cells before and after induction of terminal differentiation, during spermatogenesis in mice as well as in hepatocytes during aging, liver regeneration and carcinogenesis. In addition, we are studying the expression of the different members of the histone gene family by means of recently discovered histone mutants.