Primitive neuroectodermal tumors (PNETs) are screened by FISH with a distal 17p13.3 cosmid and a 17q25 cosmid to identify tumors with a 17p deletion. Atypical teratoid/rhabdoid tumors and PNETs without a 17p deletion are screened by FISH with a series of cosmids from 22q11.2. PNETs are also screened by interphase FISH with cosmids from chromosome 6 to identify tumors with deletions.