"Making repairs using CRISPR harnesses a cell’s own DNA repair machinery to correct genes. The technology guides a cutting protein to a particular site on the DNA molecule, chopping it open. If a DNA 'repair template' is provided—in this case a correct version of the beta-globin gene—the DNA will mend itself using the healthy sequence.
The Chinese group says that among the problems they encountered, the embryo sometimes ignored the template, and instead repaired itself using similar genes from its own genome, 'leading to untoward mutations.'
Huang said he stopped the research after the poor results. 'If you want to do it in normal embryos, you need to be close to 100 percent,' Huang told Nature News. 'That’s why we stopped. We still think it’s too immature.'"
See also: http://www.technologyreview.com/news/536021/scientists-call-for-a-summit-on-gene-edited-babies/