---
title: X-chromosome counting markers
description: The X chromosome counting markers define non-variable sequences present on the X chromosome and an autosomal chromosome that are amplified using identical primers.
---

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# [X-chromosome counting markers](https://devyser.com/blog/x-chromosome-counting-markers)

 Written by [Admin](https://devyser.com/blog/author/admin) | Oct 3, 2022 8:52:00 AM

The X chromosome counting markers define non-variable 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.

The Devyser QF-PCR kits includes several X-chromosome counting markers for reliable detection of Turner syndrome.

The inability of [STR marker](https://www.devyser.com/knowledge-hub/str-markers) 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).

[View full post](https://devyser.com/blog/x-chromosome-counting-markers)

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