The H I family is the most divergent subgroup of the highly conserved class of histone proteins [Cole: Int J Pept Protein Res 30:433-449, 19871. In several vertebrate species, the H I complement comprises five or more subtypes, and tissue specific patterns of H I histones have been described. The diversity of the H I histone family raises questions about the functions of different H I subtypes and about the differential control of expression of their genes. The expression of main type H I genes is coordinated with DNA replication, whereas the regulation of synthesis of replacement H I subtypes, such as H I " and H5, and the testis specific H I t appears to be more complex. The differential control of H I gene expression is reflected in the chromosomal organization of the genes and in different promoter structures. This review concentrates on a comparison of the chromosomal organization of main type and replacement H I histone genes and on the differential regulation of their expression. General structural and functional data, which apply to both H I and core histone genes and which are covered by recent reviews, will not be discussed in detail.1994 WtIey-Liss, Inc