Helicases are proteins that use the energy of ATP hydrolysis to open double-stranded DNA, RNA, or RNA-DNA hybrids into two single strands. Based upon sequence analysis, at least 12 helicases exist in Escherichia coli. We know that these proteins play important roles in DNA replication, recombination, repair, and transcription, as well as in RNA processing. Recent crystallographic studies have revealed a highly conserved catalytic core in the helicases, shared with the RecA protein and the F1-ATPase. However, evidence suggests that the functional divergence may be large.