As of my writing on April 12, 2018, there are 24,506 known or suspected human genes out of roughly 3 billion base pairs in the reference sequence of the human genome. While the bulk of these were identified during the course of the Human Genome Project (HGP), which ran from 1990–2003, a majority of the 5,000 or so with a well-characterized clinical phenotype—a genetic trait visible in human anatomy and physiology with consequences for human disease manifest above the cellular level—were cataloged beginning in the 1960s, long before genetic sequencing was possible. Medical geneticists worked to identify heritable traits in study populations that manifested unambiguously in family lineages. They set up clinics around the world and established sections in academic hospitals. In a discipline that was still marginal to mainstream medicine and tainted by its incomplete severance from eugenics, breaking apart old categories and multiplying new ones became a legitimation strategy, one that required physicians and counselors across the country to be on the same page.