The 3' end of the invading strand is extended by an unknown DNA polymerase and the extended strand is then ligated back to the original homolog, generating a double Holliday junction. MSH4 and MSH5 form heterodimers which bind Holliday junctions and, in the presence of ATP, slide along the parental duplexes (Bocker et al. 1999, Snowden et al. 2004, Snowden et al. 2008). MSH4 is present at hundreds of meiotic nodules during late zygotene but only about 10% of these nodules become crossovers (Oliver-Bonet et al. 2005). Bloom Syndrome protein (BLM) and Topoisomerase IIIa (TOP3A) are also present and may promote homologous recombination repair without crossing over (Johnson et al. 2000, Wu et al. 2000).