Structural abnormalities often arise from errors in homologous recombination. Early methods that targeted genes at certain sites within a genome relied on homologous recombination. Other current studies focus on specific proteins involved in homologous recombination as well. There are multiple sources of genetic variation, including mutation and genetic recombination. This is the stage when homologous recombination, including chromosomal crossover (crossing over), occurs. Mre11 has nuclease activity, but it seems to be involved in homologous recombination, not NHEJ. In the final step there is crossover by means of RecA-dependent homologous recombination. Other factors, like genetic recombination and mutation rate, can also determine SNP density. T4 proved to be orthologs for genes essential for recombination in prokaryotes, eukaryotes and archaea. BCR and TCR are both highly variable antigen receptors diversified by somatic V(D)J recombination. However, it also depends on the allelic status of the genes undergoing recombination. Evidence from studies of "S. cerevisiae" bear on the adaptive function of meiosis and recombination. An EA uses mechanisms inspired by biological evolution, such as reproduction, mutation, recombination, and selection. However, there are studies showing evidence of recombination in mitochondrial DNA. Changed pieces of information due to recombination and mutation are randomly chosen.