Giorgio Vasari, the earliest and best-known (though not always the most accurate) biographer of Renaissance Italian artists, notes that Leonardo painted a portrait of Francesco del Gioncodo's wife. As I said, Vasari is not the most accurate of sources, because he relied a great deal on hearsay for his biographies, many of which were written about artists who were already dead (and who had sometimes lived and worked in different city-states). He also had an agenda, namely the elevation of Michelangelo over his contemporaries (Vasari was an ardent follower of Michelangelo's late style, sometimes called Mannerism). Nevertheless, most art historians had always considered this to be that portrait, although there were a number of competing theories around until 2005, when a German scholar found a marginal note by one of Leonardo's contemporaries in an unrelated book. This person noted that Leonardo was then at work on a portrait of Lisa del Giocondo.