This principle is based on observations by Ahlberg as early as in 1965 that bleeding phenotype and joint status are strikingly different between severe and moderate-to-severe hemophilia patients with only minimal baseline FVIII level differences (<0.01 vs. 0.01–0.05 IU/mL).3To achieve these FVIII/FIX trough levels during prophylaxis, FVIII/FIX concentrates are mostly prescribed according to body weight.2Remarkably, it is still not usual clinical practice to standardly measure and monitor trough FVIII/FIX levels when no bleeding occurs. This evidence concerns the gene F8 and hemophilia.