The Mona Lisa is regarded by many as the most valuable painting the world. Created by polymath Leonardo da Vinci at the beginning of the 16th century, the work is often used to represent art itself.
In 1963, the work was brought to America, thanks to the efforts of First Lady Jackie Kennedy. It was the first time the work had left the French Louvre museum in 50 years, having been stolen previously by an Italian nationalist (Picasso had been suspected of the theft at one stage).
The exhibition of the painting drew half a million viewings during its visit, and is often regarded at the moment Americans gained a real interest in the art of the old masters.
Now some who were growing up at the time, notably Microsoft co-founder Paul Allen, have collections of old master paintings worth millions (though he is extremely secretive about which works he has).
His co-founder and fellow billionaire Bill Gates, (whose collections we looked at in a newsletter) also bought the Codex Leicester for $30.8m at a New York auction, a particularly valuable collection of da Vinci's writings which he scanned and made available digitally.
In general, Leonardo's work remains phenomenally valuable. One of the great stories of 2009 was that of a painting found to be a work by da Vinci, which increased its estimated value by over 5,000 times to $100m.
As for the Mona Lisa itself, it remains by estimate the most valuable painting in the world. Assessed for insurance in 1962 it was given a notional price tag of $100m. Whilst other paintings have now sold for more than this (notably a Jackson Pollock for $140m), a comparable 2010 estimate would be around $700m.