Scientists developed a new version of a gene drive that allows the spread of specific, favorable genetic variants, also known as 'alleles,' throughout a population. The new 'allelic drive' is equipped with a guide RNA that directs CRISPR to cut undesired variants of a gene and replace it with a preferred version. Using a word processing analogy, CRISPR-based gene drives allow scientists to edit sentences of genetic information, while the new allelic drive offers letter-by-letter editing.