In pediatric cancers, all of which are defined as rare cancers, PTEN activity loss is seen in T-lineage or B-lineage acute lymphoblastic leukemia (T-ALL or B-ALL), osteosarcoma, high grade glioma, medulloblastoma, pilocytic astrocytoma, adrenocortical carcinoma, rhabdomyosarcoma, and pediatric thyroid carcinomas [96,97,98,99,100]. The gene discussed is PTEN; the disease is osteosarcoma.