In addition to previously characterized testis-specific and somatic H2B histone genes, two somatic histone H2B genes, hereafter called sH2B-2 and sH2B-3, were isolated from a rat genomic library, genomic organization was determined, and their promoter was sequenced. Like many other H2B genes, sH2B-2 gene was closely linked to H2A gene whereas H3 gene was located upstream of sH2B-3 gene. Deletion and mutation analysis of 5' sequence fused to CAT reporter gene revealed that the interaction between CCAAT and octamer binding factors is important for S-phase-specific activation of sH2B-3 gene.