['The neutral 807 C/T (Phe224) polymorphism (807 C/T polymorphism) of the glycoprotein (GP)Ia gene has been recently associated with the number of GPIa molecules on the platelet surface. The association of the number of GPIa molecules with other GPIa polymorphisms, such as HPA-5 (Glu/Lys505) (HPA-5 polymorphism), involved in alloimmune thrombocytopenias is unknown.', 'The association of the HPA-5 polymorphism with the number of GPIa molecules on the platelet surface in 159 white blood donors was investigated. The genetic linkage between the HPA-5 and the 807 C/T polymorphisms in 316 individuals was also determined.', 'Both the 807 C/T and HPA-5 polymorphisms correlate with the number of GPIa molecules on the platelet surface. The 807 T and HPA-5b alleles are associated with increased numbers of GPIa molecules on the platelet surface. Moreover, the HPA-5b allele is genetically linked to 15.8 percent of the 807 C alleles. Therefore, the number of GPIa molecules on the platelet surface is dependent on both GPIa polymorphisms as follows: 807 T/T, HPA-5 a/a > 807 C/T, HPA-5 a/b > 807 C/T, HPA-5 a/a > 807 C/C, HPA-5 a/b > 807 C/C, HPA-5 a/a.'] Is the number of platelet glycoprotein Ia molecules associated with the genetically linked 807 C/T and HPA-5 polymorphisms?