Factor-IX Gla/Ds Red expression system. (A) Vector with doxycycline-inducible bidirectional promoter directing expression of cytoplasmic DsRed to assess induction, and chimeric factor-IX Gla domain-containing protein (F9CH) to measure carboxylation activity. Tet-On cells containing this vector are referred to as the “C3” cell line. (B) Chimeric factor-IX Gla protein contains prothrombin signal and pre-pro-peptide directing γ-carboxylation of the factor-IX Gla domain. APC-labeled antibody (GMA-001) specifically binds the γ-carboxylated factor-IX Gla domain.