[Fragile site Xq27 and metabolism of monocarbons].

After confirming the increase of the fragility of the site Xq27 by methotrexate and its decrease by 5-formyltetrahydrofolate, it is observed that L-serine, 5-amino-levulinate and L-4-hydroxy-proline at millimolar concentrations, reduce considerably the frequency of the anomaly in lymphocyte cultures. The possible relationships between this chromosomal marker, the one carbon cycle,and mental deficiency of the patients, are discussed.