The Xg blood group system: no longer forgotten

Abstract This update of the Xg blood group system (Johnson NC. XG: The forgotten blood group system. Immunohematology 2011;27:68–71) notes the identification of a cis- regulatory element of both XG and CD99 expression, remarkably by two independent groups during 2018, and confirmed by another in 2019. A single nucleotide change at the XG locus (rs311103) abolishes GATA1 binding and suppresses both XG and CD99 . The last blood group system to resist elucidation of its genetic basis was thereby resolved. Soon afterwards, it was discovered that the rare anti-Xg a response, mainly seen in men, is produced by individuals primarily carrying a large deletion in the X chromosome that truncates XG and leads to the Xg null phenotype.