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Hamato Yoshi was a ninja in the service of the Foot Clan. Unidentified reasons forced him to flee Japan and take up residence in the Sewers of New York City, where his only friends were the resident rat population.
One day, a bowl of pet turtles was shattered on an open sewer grate, dropping the turtles down into the sewers below. Some time later, both Yoshi and the turtles were contaminated by a mutagen ooze, which anthropomorphized the turtles and transformed Yoshi into a rat-man, who the Turtles now called Splinter. Splinter trained the Turtles in the art of ninjutsu.