Mutants may also be generated using recombination of multiple homologous genes. A characteristic central feature of meiosis is recombination between homologous chromosomes. In meiosis and mitosis, recombination occurs between similar molecules of DNA (homologous sequences). Meiosis involves the intimate pairing of homologous chromosomes and recombination between them. The degree of sequence similarity between homologous genes is called conserved sequence. Structural abnormalities often arise from errors in homologous recombination. DNA strand annealing is a key step in pathways of homologous recombination. Cells and organisms with pairs of homologous chromosomes are called diploid. Early methods that targeted genes at certain sites within a genome relied on homologous recombination. However pairing is still sufficient to allow some recombination between homologous chromosomes. This pathway will eventually recruit the necessary proteins for homologous recombination repair. Other current studies focus on specific proteins involved in homologous recombination as well. This enzyme is extremely important in the process of homologous recombination. Recombinases are capable of pairing oligonucleotide primers with homologous sequence in duplex DNA. Techniques that rely on homologous recombination have been used to engineer new proteins.