23&me tests the X Chromosome and includes it in their raw atDNA file.
For male to male matches, the threshold is 1cM and 200 SNPs.
For female to male matches, the threshold is 6cM and 600 SNPs.
For female to female matches, the threshold is 6cM and 1,200 SNPs.
FTDNA tests the X chromosome but only identifies it as a match after meeting the atDNA threshold of 1 segment of at least 5.5cMs. At that point, the X match must share 1cM and 500 SNPs.
Women have 2 X chromosomes. The one from their father is inherited unchanged. There is no recombination.
The X chromosome is comprised of 150 million base pairs and 2,000 genes.
The X tests 17,000 - 20,000 SNPs.
For males, matches on the X chromosome must be a maternal match. Sisters sharing a father will always share an entire X chromosome.
Unidentical siblings with same mother:
1. Can share the entire maternal X-chromosome from either maternal grandmother or maternal grandfather
2. Can share 1 or more portions of the X
3. Can share no maternal X-DNA
If the user has not identified a gender for their kit on GEDmatch, the following test can be done with 23andMe kits only:
Take a 23andMe kit for a female and compare it against a 23andMe kit for an unknown gender using the X one-to-one comparison tool, with the graphics bar turned on. If the color green is seen in the results, the kit with the unidentified gender is that of a female.
For example, Kit M085827 is for a female. Kit M202956 is unidentified. There is no color green in the X comparison performed, so therefore the gender of M202956 is male.
 Blaine T. Bettinger, The Family Tree Guide to DNA Testing and Genetic Genealogy (Cincinnati OH : Family Tree Books, 2016), 103, 23andme.
 Bettinger, The Family Tree Guide, 104, Family Tree DNA.
 Ibid., 125, The Unique Inheritance of X-DNA.
 Ibid., 124, The Unique Inheritance of X-DNA.
 Ibid., 128-129, How the Test Works.
 Blaine T. Bettinger and Debbie Parker Wayne, Genetic Genealogy in Practice (Arlington, VA. : National Genealogy Society, 2016), 103, Advantages, Limitations, and Test Strategies for X-DNA.
 Bettinger and Wayne, Genetic Genealogy in Practice, 104, Advantages, Limitations, and Test Strategies for X-DNA.