Each new development inches toward designer babies
by Julie Fidler, http://naturalsociety.com:
A group of scientists in Oregon have edited the genes of human embryos for the first time in the United States, using CRISPR-Cas9, a cut-and-paste gene-editing tool. 
The experiments were conducted by Shoukhrat Mitalipov and colleagues at Oregon Health & Science University in Portland. The single-celled embryos Mitalipov edited were discarded after the experiments to ensure they could not become too developed.
Live Science explains how the CRISPR/Cas9 gene-editing tool works:
“The CRISPR/Cas9 gene-editing system is a simple “cut and replace” method for editing precise spots on the genome. CRISPRS are long stretches of DNA that are recognized by molecular “scissors” called Cas9; by inserting CRISPR DNA near target DNA, scientists can theoretically tell Cas9 to cut anywhere in the genome. Scientists can then swap a replacement gene sequence in the place of the snipped sequence. The replacement sequence then gets automatically incorporated into the genome by natural DNA repair mechanisms.”
Source: Daily Mail
CRISPR was used by scientists in China in 2015 to edit several human embryos that had severe defects. Like the scientists in Oregon, the team in China discarded the embryos before they could become too developed.
The Chinese technique led to genetic changes in a few of the embryos, and in a few cases, CRISPR sometimes snipped out the wrong place in the DNA.