A very rare, fast-growing cancer that usually forms in the tissues of the central nervous system (CNS), such as the cerebellum (the part of the brain that controls movement and balance) or brain stem. ATT/RHTs may also form in areas of the body outside the CNS, such as the kidney, but this is rare. Most of these types of tumors are caused by mutations (changes) in the tumor suppressor genes SMARCB1 or SMARCA4, which make proteins that help control cell growth. ATT/RHTs usually occur in children aged 3 years and younger, but they can also occur in older children and adults. Also called AT/RT and atypical teratoid/rhabdoid tumor.