Kao CS, et al. - Researchers here studied chromosome 12p copy number in 34 diverse mediastinal germ cell tumors (GCTs). Further, they correlated the results with morphology and follow-up to gain insight into possible pathogenesis. For this work, they analyzed 4 prepubertal (below 12 y) children (3 females and 1 male), 7 postpubertal females (14 to 52 y) and 6 postpubertal males (12 to 40 y old) with pure, previously untreated teratomas; 15 were mature and 2 had low-grade immaturity. As per results, 2 separate pathways seemed involved in mediastinal teratomas development. These teratomas arise as pure neoplasms from a nontransformed precursor cell in children, women and some men and, therefore, lack 12p copy number increase, show no cytologic atypia, often have organoid morphology and are benign. In postpubertal males, they observed common 12p copy number increase, uniform atypia, infrequent organoid structures and malignant behavior, supporting that pure teratomas after chemotherapy derive from a malignantly transformed precursor cell. Further, only the benign group displayed organoid pancreatic differentiation and the malignant teratomas displayed neuroglia more commonly.