What disease or hereditary gene disorder affected Benjamin Buttons, played by Brad Pitt, in the move "the curious case of Benjamin Buttons?"

What disease or hereditary gene disorder affected Benjamin Buttons, played by Brad Pitt, in the move "the curious case of Benjamin Buttons

Please be patient, we are ordinary people who volunteer our time here to help others. Considering this is the holiday season, many volunteers may be busy with family and friends.

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I believe it is a work of fiction.