Metazoan histone mRNAs are unique in that they lack a 3'-polyadenylated tail, but instead end in a conserved stem loop that is bound by SLBP (stem-loop binding protein). SLBP is required for efficient histone mRNA synthesis and translation. Removal of SLBP by RNA interference causes an increase in the number of cells in S-phase and this effect can be reversed by expressing an exogenous SLBP resistant to the small interfering RNA. Cells with decreased SLBP levels progress slowly through S-phase when released from a double-thymidine block. Thus SLBP is required for efficient DNA replication probably because a decreased ability to assemble chromatin results in a decrease in the rate of DNA replication.