Fyx is associated with two missense point mutations in its gene and can be detected by PCR-SSP.

The Duffy blood group antigens are encoded by the Duffy gene with its three major alleles: Fy*A (Fya+), Fy*B (Fyb+), and a nonexpressed Fy*Fy (Fya-b-), which is most commonly found among black people. Additionally, a fourth allele, Fyx, is found among white people and defined as weak Fyb not detectable by all anti-Fyb. Three polymerase chain reactions (PCRs) using sequence-specific priming (SSP) for detection of the major FY alleles were developed. Eighteen Fy(a-b-) samples of Tanzanian origin were correctly typed and of 300 random donors of Caucasian origin with known Fy phenotype, only four out of 59 Fy(a+b-) donors showed the discrepant DNA-type Fy(a+b+). Serologic reinvestigation by adsorption and elution techniques confirmed weakly expressed Fyb antigen in these cases and DNA sequencing of the entire Duffy gene revealed identical point mutations in all of them. Specific PCR reactions were used to reinvestigate the C265T (Arg89Cys) and G298A (Ala100Thr) substitution in the 300 samples. A298 was found to be present in both FY*X and FY*B alleles, pointing to an allelic variation among FY*B alleles. T265 was encountered exclusively in FY*X and is thought to be FY*X specific. Combining the T265 specific reaction with the three PCR-SSPs described above, we were able to correctly DNA-type all phenotypes investigated in our study.