Isleworth Mona Lisa
According to Leonardo's early biographer Giorgio Vasari, Leonardo had started to paint "Mona Lisa" in 1503, but "left it unfinished". However, a fully finished painting of a "certain Florentine lady" surfaced again in 1517, shortly before Leonardo's death and in his private possession. The latter painting almost certainly is the one that now hangs in the Louvre. Based on this contradiction, supporters of the authenticity of the "Isleworth Mona Lisa" claim it is the unfinished "Mona Lisa", made at least partially by Leonardo, and the Louvre "Mona Lisa" a later version of it, made by Leonardo for his own use.
Isabella d'Este
Isabella d'Este has been proposed as a plausible candidate for Leonardo's "Mona Lisa" of c. 1503–1506, usually considered a portrait of Lisa del Giocondo. (Lisa was the wife of a merchant in Florence and Giorgio Vasari wrote of her portrait by Leonardo – it remains open whether this is the portrait now known as the 'Mona Lisa'.) Evidence in favor of Isabella as the subject of the famous work includes Leonardo's drawing 'Isabella d'Este' from 1499 and her letters of 1501–1506 requesting the promised painted portrait; further arguments are the mountains in the background and the armrest as a Renaissance symbol for a portrait of a sovereign.