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-Xga 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 Xgnull phenotype.