Though there a few genes that are correlated to stuttering, it is very hard to identify why these genes can cause stuttering and basically impossible to modify these genes in a living person. As the cells in our brain already have our normal genome, the only way to make someone with brain cells that have a modified genome would be to apply the modifications to a fertilized human embryo in a lab, and so would be done before the person is born. This is illegal to do. Also, we have no idea which modifications to make to produce someone who does not stutter without repeated trial and error, and as it’s illegal to modify human embryos, this is not going to happen. Even if you could do it illegally, you would only know if you’re successful for each trial run after waiting many years for that person to develop, which could make this a decades long experiment to conduct. Genes can also have many functions, and we don’t know how these other functions will be affected if we modify these genes. Also, many people stutter even though no one in there family has a stutter, which would make trying to fix stuttering through gene modification basically useless for these people.