The painting was cleaned of its varnish (which turns yellow when exposed to light) and it turned out to be a copy of the famous portrait, painted at the same time as the original and probably in Leonardo's workshop. The author of the copy may have been one of his students. The copy from the Prado is in cool colors, while the coloring of the Mona Lisa from the Louvre is entirely different - it is supposed to be due to the varnish.
