Besides the adolescent girl who had heparin-induced thrombocytopenia type 2 on unfractionated heparin, three of the five children on prophylactic LMWH therapy showed reduced 4-h anti-factor Xa activity (0·1, 0·25, and 0·34 IU/mL) at the time of recurrence. This evidence concerns the gene F10 and thrombocytopenia 2.