GP6 and Marfan syndrome: The levels of P-selectin exposure and GPVI dimerization were low on non-activated platelets from the two groups and not significantly different (Table 1) whereas the levels of GPVI expression as measured using the 3J24 antibody were not different between the two groups of patients and between Marfan syndrome patients and healthy controls [83.37 ± 12.8% vs 87.9 ± 11.6% (mean ± SD)].