After leaving GoCollege.com, Greenspan became semi-retired and returned to working on his family history. In 1999, he began work on his mother's Nitz lineage. When faced with an unsurmountable obstacle using standard genealogical methods in his work, he remembered two cases of genetics being used to prove ancestry that had recently been covered by the media. One was a study by
University of Arizona researchers showing that many
Cohen men from both
Ashkenazic and
Sephardic groups share the same
Y-Chromosome pattern of
markers, the
Cohen Modal Haplotype. The other was a
DNA study showing that male descendants of US President
Thomas Jefferson with the Jefferson surname, and male descendants of his freed slave
Sally Hemings, shared the same Y-Chromosome markers and a recent common ancestry. Greenspan had Nitz cousins in California, and he had discovered someone in
Argentina with the same ancestral surname and the same ancestral location in Eastern Europe. Wishing to use the same method of DNA comparison for his own genealogy, he contacted Dr. Michael Hammer at the University of Arizona. Their conversation inspired Greenspan to start a company dedicated to using genetics to solve genealogy problems, Family Tree DNA. As personal genetic ancestry and genealogy industries grew, and so did Family Tree DNA. In 2004, Greenspan and Blankfeld hosted the first conference for advocates in the field. However, the industry faced ethics and infrastructure challenges. ==Gene by Gene==