The intragenic (FMR‐1) probe pE5.1 was used for DNA analysis in fragile X families. With this probe fragments of altered size can be detected in female carriers, affected individuals and transmitting males. the length of the altered fragments was found to vary from one generation to another as well as between sibs. This instability of the DNA detected by pE5.1 was also seen in peripheral blood within single individuals. These phenomena are illustrated by 4 exemplary families segregating the fragile X syndrome. We demonstrate the diagnostic contribution of intragenic analysis to carrier detection as well as the identification of normal transmitting males carrying premutations. One of the families illustrates the passage of a premutation to a male through 2 generations.