Activated factor IX (FIXa) binds to anionic phospholipid surfaces with high affinity (Mertens K & Bertina RM 1984, Mertens K et al. 1984; Greengard JS et al. 1986). This binding is mediated by the N-terminal gamma-carboxyglutamic acid (Gla) domain of FIX in the presence of Ca2+ (Jacobs M et al., 1994; Freedman SJ 1996; Huang M et al., 2004). On the surface, FIXa associates with its cofactor FVIIIa, forming the FIXa:FVIIIa complex, known as "tenase" or "X-ase". The FIXa:FVIIIa complex catalyzes the conversion of factor X (FX) to its activated form, FXa (reviewed by Stoilova-McPhie S 2021; Childers KC et al., 2022; Bos MHA et al., 2024). Studies using physiologic surfaces provide evidence for coordinated binding interactions of the enzyme, cofactor and substrate to discrete surface structures. For example, the presence of both (active site-modified) factor IXa and factor X increased both the number and the affinity of binding sites on activated platelets for factor VIIIa (Ahmad SS et al. 2000). However classical receptors for the constituents of factor Xase have not been identified (Fay PJ 2004).