Nijmegen breakage syndrome (NBS) is a rare inherited disorder in which the body's DNA is unstable and tends to break. Most infants with NBS have unusually small heads at birth, or become more obvious or severe as the infant grows. Growth is usually slow during the first few years of life which leads to shorter than average height as the individual ages. Development and intellect appear to progress normally for the first few years of life but then begin to decline, causing mild to moderate intellectual disability. Individuals with NBS experience recurrent infections due to a malfunctioning immune system and are also at an increased risk of developing cancer. Most women with NBS will have premature ovarian failure and infertility issues. There is no cure for NBS; however medical surveillance and care may help to improve some symptoms and overall condition of life. Large doses of radiation are lethal for individuals with NBS so appropriate treatment for cancers must be discussed. NBS is caused by pathogenic variants in the NBN gene.