The antibiotic heliquinomycin, which inhibits cellular DNA replication at a half-maximal inhibitory concentration (IC50) of –4lm, was found to inhi-bit the DNA helicase activity of the human minichromosome maintenance (MCM) 4/6/7 complex at an IC50value of . In contrast, 14 lmheliqui-nomycin did not inhibit significantly either the DNA helicase activity of the SV40 T antigen and Werner protein or the oligonucleotide displacement activity of human replication protein A.