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.
Speculations about Mona Lisa
In January 2012 Museo del Prado in Madrid announced that it had discovered and almost fully restored a copy of the painting by a pupil of Leonardo, very possibly painted alongside the master. The copy gives a better indication of what the portrait looked like at the time, as the varnish on the original has become cracked and yellowed with age.