with ATLD who are well described differ from those with A–T by the absence of telangiectasia, normal immunoglobulin levels, a later onset, and a slower progression of the symptoms. Because of its rarity, it is not yet known whether or not ATLD carries an increased risk to develop cancer. Because those mutations of Mre11 that severely impair the MRE11 protein are incompatible with life, individuals with ATLD all have some partial function of the Mre11 protein, and hence likely all have their own levels of disease severity. Nijmegen breakage syndrome (NBS) is a rare genetic disorder that has similar chromosomal instability to that seen in people with A–T, but the problems experienced are quite different. Children with NBS have significant microcephaly, a distinct facial appearance, short stature, and moderate cognitive impairment, but do not experience any neurologic deterioration over time. Like those with A–T, children with NBS have enhanced sensitivity to radiation, disposition to lymphoma and leukemia, and some laboratory measures of impaired immune function, but do not have ocular telangiectasia or an elevated level of AFP. The proteins expressed by the hMre11 (defective in ATLD) and Nbs1 (defective in NBS) genes exist in the cell as a complex, along with a third protein expressed by the hRad50 gene. This complex, known as the MRN complex, plays an important role in DNA damage repair and signaling and is required to recruit ATM to the sites of DNA double strand breaks. Mre11 and Nbs1 are also targets for phosphorylation by the ATM kinase. Thus, the similarity of the three diseases can be explained in part by the fact that the protein products of the three genes mutated in these disorders interact in common pathways in the cell. Differentiation of these disorders is often possible with clinical features and selected laboratory tests. In cases where the distinction is unclear, clinical laboratories can identify genetic abnormalities of ATM, aprataxin and senataxin, and specialty centers can identify abnormality of the proteins of potentially responsible genes, such as ATM, MRE11, nibrin, TDP1, aprataxin and senataxin as well as other proteins important to ATM function such as ATR, DNA-PK, and RAD50. Management Ataxia and other neurologic problems There is no treatment known to slow or stop the progression of the neurologic problems. Immune problems All individuals with A–T should have at least one comprehensive immunologic evaluation that measures the number and type of lymphocytes in the