Random mutagenesis is one of the most effective methodologies to generate variant libraries for directed protein evolution. Indeed, this approach requires no structural or mechanistic information and can uncover unexpected beneficial mutations. Here, we describe a new random mutagenesis method based on the use of human error-prone DNA polymerases (pol beta, pol eta and pol iota). This approach allows the random introduction of mutations through a single replication step followed by a selective PCR amplification of the replicated mutated sequences. The libraries generated using this methodology display different mutation rates and complementary mutational spectra. By taking advantage of the mutation bias of naturally highly error-prone DNA polymerases, MutaGen thus appears as a very useful tool for gene and protein randomization.