GIST48 and GIST882 cells were established in the Fletcher laboratory (DFCI). All other cells were obtained from ATCC. Etv1−/− mice, with targeted deletion of the ETS domain, was obtained from the Jessell laboratory (Columbia) and CB17-SCID mice was from Taconic. Antibody sources are: ETV1, ANO1, PGP9.5 (Abcam), KIT for WB, P-Tyr703-KIT (Cell Signaling), P-Tyr204-ERK, GAPDH (Santa Cruz), and anti-mouse Kit for IF (clone ACK2, E-Biosciences).