The Devyser QF-PCR kits includes several X-chromosome counting markers for reliable detection of Turner syndrome.
The inability of STR marker analysis to distinguish subjects who are homozygous or monosomic is a major shortcoming when testing for sex chromosome abnormalities. When STRs specific for chromosome X are used, some samples from normal XX females may show homozygous QF PCR patterns, indistinguishable from those produced by samples with a single X, as in Turner syndrome.
The T1, T2 and T3 markers are non-polymorphic (non-STR) X chromosome counting markers that may be used to determine the number of X chromosomes when monosomy X is suspected. These markers define sequences present on the X chromosome and an autosomal chromosome that are amplified using identical primers. The amplified marker fragments are separated according to length and the X chromosomal copy number is determined by fragment area ratio calculation.
In a normal female an X chromosome counting marker area ratio of 1:1 is expected. In normal males and females with monosomy X a 2:1 ratio is expected (see figure below).