Susceptibility to platelet-type bleeding disorder-13 (BDPLT13) is due to a defective thromboxane A2 receptor on platelets. The susceptibility is inherited in an autosomal dominant pattern, but clinical features, including mild mucocutaneous bleeding, occur only in the presence of a 'second hit' affecting platelet function; this second hit may be either in the TBXA2R gene or in another gene affecting the coagulation cascade (summary by Mumford et al., 2010).