Ullrich-Turner syndrome (UTS) is frequently associated with monosomy X but may also occur with structural aberrations of the X and the Y chromosomes. It has been hypothesized that the ribosomal protein genes RPS4X and RPS4Y play a critical role in the prevention of UTS. Individual patients with a 46,X,i(Xq) karyotype cannot be differentiated phenotypically from 45,X UTS patients and carry three gene copies of RPS4X. Since haploinsufficiency of one or several gene(s) is thought to cause the UTS phenotype, direct assessment of RPS4X expression levels in these patients should establish whether RPS4X is involved in UTS. We have investigated fibroblasts of four 46,X,i(Xq) UTS patients with typical symptoms and a non-mosaic chromosome complement, and have found significantly increased RPS4X mRNA levels in all patients. Based on our results, we conclude that haploinsufficiency of RPS4X is not the cause of UTS.