Evidence for differential gene expression during the cell cycle and approaches for studying cell-cycle-stage specific gene expression are summarized. Attention is focused on regulation of histone gene expression during the cell cycle of continuously dividing cells and after stimulation of nondividing cells to proliferate. The level(s) at which control of histone gene expression occurs and the possible involvement of chromosomal proteins in the regulation of histone gene expression are discussed. The preparation of cloned human histone sequences and their use in studying the structural and functional properties of human histone genes are presented. Index Entries: Cell cycle, gene regulation during; gene regulation, during the cell cycle; regulation of specific genes, during the cell cycle; DNAs, homologous, and histone gene expression; cloned DNAs, and histone gene expression; histone gene expression; gene expression, histone; cloned human histone sequences.