IF AN RNA OR PROTEIN MOLECULE is damaged, the cell will typically degrade the entire molecule and replace it with a new one. But cells cannot do this with DNA. A chromosome is too big and the copy number is too low, so it is far more efficient to repair the chromosome than to replace it. If only one DNA strand is damaged, the other may be used as a template for repair; alternatively, if both strands are damaged, the homologous chromosome may be used. DNA repair involves DNA synthesis, sometimes of just 1 nucleotide, and sometimes of thousands or more, depending on the type of damage and the method of repair (Alberts et al. 2002). This repair-associated DNA synthesis has given rise to multiple forms of drive. On the one hand, there is biased gene conversion (BGC). Like other forms of drive, it is a process by which a heterozygote transmits one allele more frequently than the other. This drive does not arise because the gene encodes a protein that acts to bias inheritance; rather, drive arises as a byproduct of biases in the mechanisms of DNA repair. This is therefore a more passive form of drive. It is also relatively weak, with effective selection coefficients of 1% or less. But BGC has the potential at least to be pervasive, with effects across the entire genome, and several lines of evidence suggest it can play a major role in shaping the composition of eukaryotic genomes. It is also likely to have played an important role in the evolution of the meiotic machinery.