The resulting break, called a DNA double strand break (DSB), is then fixed by the cell’s natural DNA repair machinery. The SRY gene carries the instructions for a protein called the sex-determining region Y, which is involved in male sexual development. To insert the SRY gene into the H11 locus, the team designed a HMEJ donor template, with the H11 CRISPR target sequence on either side of the homology arms. In initial experiments, only around 40% of the bovine embryos were found to have the SRY insertion. This meant the team needed to confirm whether the gene insertion had been successful before the embryo was transferred to a surrogate cow.