May the FORS be with you: a system sequel.

This article is an update of the review of the FORS system published in Immunohematology in 2017 (Hult AK, Olsson ML. The FORS awakens: review of a blood group system reborn. Immunohematology 2017;33:64-72). This update incorporates the most recently presented knowledge on this still enigmatic system and its genetic, enzymatic, and immunological aspects. Further insight into the genetic variation and allele frequencies of the GBGT1 locus has been reported, and screening studies regarding the prevalence of naturally occurring anti-FORS1 in human plasma have been performed and presented. More basic knowledge on the specificity of the gene product, the Forssman synthase, has been obtained in several detailed studies, and its relation to the homologous ABO gene has been investigated. Taken together, we summarize recently added information about the carbohydrate-based FORS blood group system (International Society of Blood Transfusion number 031).