Radiolab covers the strange saga of Marvel Comics's fight against the US customs authority over whether X-Men dollies were "dolls" or "toys" — the difference being that dolls (which are defined as characters that represent humans) are taxed at twice the rate of "toys." The case turned on whether a mutant was, indeed, human, or whether they were monsters. Despite Professor X's long advocacy for the essential humanity of mutants, his corporate owners argued that he and his cohort were mere monsters (for tax purposes).
Reporter Ike Sriskandarajah tells Jad and Robert a story about two international trade lawyers, Sherry Singer and Indie Singh, who noticed something interesting while looking at a book of tariff classifications. "Dolls," which represent human beings, are taxed at almost twice the rate of "toys," which represent something not human – such as robots, monsters, or demons. As soon as they read that, Sherry and Indie saw dollar signs. it just so happened that one of their clients, Marvel Comics, was importing its action figures as dolls. And one set of action figures really piqued Sherry and Indie's interest: The XMEN, normal humans who, at around puberty, start to change in ways that give them strange powers.
Here's the actual court opinion (PDF).
The solomonic court divided the mutants into varying degrees of humanness. In the human camp were the Invisible Woman, Punisher, Daredevil, U.S. Agent, Peter Parker, and Jumpsie were humans. The remainder (including the Fantastic Four) were mutants.