periphery, they rearrange special segments of their DNA [V(D)J recombination process]. This process requires them to make DSBs, which are difficult to repair in the absence of ATM. As a result, most people with A–T have reduced numbers of lymphocytes and some impairment of lymphocyte function (such as an impaired ability to make antibodies in response to vaccines or infections). In addition, broken pieces of DNA in chromosomes involved in the above-mentioned rearrangements have a tendency to recombine with other genes (translocation), making the cells prone to the development of cancer (lymphoma and leukemia). Progeric changes Cells from people with A–T demonstrate genomic instability, slow growth and premature senescence in culture, shortened telomeres and an ongoing, low-level stress response. These factors may contribute to the progeric (signs of early aging) changes of skin and hair sometimes observed in people with A–T. For example, DNA damage and genomic instability cause melanocyte stem cell (MSC) differentiation which produces graying. Thus, ATM may be a "stemness checkpoint" protecting against MSC differentiation and premature graying of the hair. Telangiectasia The cause of telangiectasia or dilated blood vessels in the absence of the ATM protein is not yet known. Increased alpha-fetoprotein (AFP) levels Approximately 95% of people with A–T have elevated serum AFP levels after the age of two, and measured levels of AFP appear to increase slowly over time. AFP levels are very high in the newborn, and normally descend to adult levels over the first year to 18 months. The reason why individuals with A–T have elevated levels of AFP is not yet known. Neurodegeneration A–T is one of several DNA repair disorders that result in neurological abnormalities or degeneration. Arguably some of the most devastating symptoms of A–T are a result of progressive cerebellar degeneration, characterized by the loss of Purkinje cells and, to a lesser extent, granule cells (located exclusively in the cerebellum). The cause of this cell loss is not known, though many hypotheses have been proposed based on experiments performed both in cell culture and in the mouse model of A–T. Current hypotheses explaining the neurodegeneration associated with A–T include the following: Defective DNA damage response in neurons which can lead to Failed clearance of genomically damaged neurons during development Transcription stress and abortive transcription including topoisomerase 1 cleavage complex (TOP1cc) dependent lesions Aneuploidy Defective response to oxidative stress characterized by elevated ROS and altered cellular metabolism Mitochondrial dysfunction