Thrombotic risks: a clarification.

To the Editor: Rogier M. Bertina has written a clear, timely, and authoritative review of factor V Leiden and other coagulation factor mutations affecting thrombotic risk (1). I was confused by the fact that the sum of genetic defects in inherited thrombophilia plus the cases with unknown cause add up to 109.7% (Table 1 of reference 1). This could occur due to multiple defects in individual patients. However, it might just be a typographical error, where the unknown cases are 20% rather than 30%. I would appreciate clarification, as the answer either reduces the number of residual unclassified cases or possibly suggests frequent overlap of thrombophilic states, which would seem to be new area of inquiry.