The H1 class of histones comprises several main-type, S-phase dependent isoforms and, in addition, a sperm-specific H1t and a peculiar subtype, H1 zero, which is confined to highly differentiated cells. In contrast to main type histone genes, the H1 zero gene expression does not strictly depend on DNA replication. Also in contrast to the other H1 subtype genes, the mammalian H1 zero gene is not included in a histone gene cluster and its mRNA differs in structure, size and mode of processing from other histone mRNAs. The regulation of expression of the H1 zero gene varies from main type H1 genes in several respects. This is manifested in the promoter structure which contains sequence elements that are also found in main type H1 promoters, but also shows regulatory motifs which appear to be involved in a developmental regulation of the H1 zero gene, such as a retinoic acid receptor binding site, which has been described in the mammalian H1 zero gene promoter.