Distinguishing features: The Mona Lisa's deep-set eyes and round face do not conflict with Duchamp's act of violence. The beard and moustache seem a completion. Duchamp said the Mona Lisa becomes a man - not a woman disguised as a man, but a real man. This hints at a different meaning from vandalism, for all the crudeness of those letters, ., which sound out the French sentence: "She has a hot arse." This is not simply an attack on the mass-produced tourist icon the Mona Lisa had become, but rather an inter-pretation of it. Sigmund Freud had psychoanalysed Leonardo's art and related the artist's inability to finish his works to the sublimation of his sexual life to art. He also argued that Leonardo was homosexual.
“This painting, which Marcel Duchamp identified as a self-portrait, was probably begun during December of 1911 in Neuilly, while he was exploring ideas for the controversial Nude Descending a Staircase, No. 2 of 1912 (Collection Philadelphia Museum of Art). In Nude (Study), Sad Young Man on a Train his transitory though acute interest in Cubism is manifested in the subdued palette, emphasis on the flat surface of the picture plane, and in the subordination of representational fidelity to the demands of the abstract composition.”
Guggenheim Collection – Artist – Duchamp – Nude (Study), Sad Young Man on a Train
French poet Guillaume Apollinaire , who had once called for the Louvre to be "burnt down", came under suspicion and was arrested and imprisoned. Apollinaire implicated his friend Pablo Picasso , who was brought in for questioning. Both were later exonerated .   Two years later the thief revealed himself. Louvre employee Vincenzo Peruggia had stolen the Mona Lisa by entering the building during regular hours, hiding in a broom closet, and walking out with it hidden under his coat after the museum had closed.  Peruggia was an Italian patriot who believed Leonardo's painting should have been returned for display in an Italian museum.