Kurtzman acted as mentor to a large number of cartoonists, such as Terry Gilliam, Robert Crumb, and Gilbert Shelton. Students of his at the School of Visual Arts included John Holmstrom, Batton Lash, and Drew Friedman. Kurtzman, and particularly his work on "Mad", is the most frequently cited influence on the movement—comics historian Mark Estren called "Mad" "the granddaddy of the underground ". In 1958, Robert Crumb and his older brother Charles self-published three issues of the "Humbug"-inspired fanzine "Foo" in 1958. The venture was not a financial success, and Crumb turned to producing comics to satisfy himself. In 1964 Kurtzman published his work in "Help!".