In the fragile X female carriers the degree of cognitive impairment appears to be correlated with activation status of the X chromosome bearing the expanded trinucleotide repeat in the promoter of the FMR1 gene. In this study we asked if the deviations from the primarily random pattern of X inactivation are related to the selection which is thought to occur against cells carrying the fragile X full mutation (FM) on the active X chromosome. A fibroblast culture derived from a 20-week FM female fetus was serially passaged. The activation ratio (AR) of the culture increased from 0.68 to 0.92 between passages 2 and 9. All higher passage cells (up to 34 passages) display an AR of 1.0, indicating complete absence of cells in which the normal X chromosome would be inactivated. Of 29 clones established from the fetal culture with AR of 0.8, 28 had no visible 5.2-kb band on Southern blots indicating that these 28 clones consisted entirely of cells with FM on their inactive X chromosome. Only a single clone carried the FM on its active X chromosome. The figure of 1 of 29 is much lower than our expectation based on the AR of mass culture. Therefore cloning and serial cultivation indicate the possibility of selection depending on the activation status of the expanded X chromosome in fetal FM female fibroblasts. Am.