Haptoglobin-related protein (HRP) is present in human serum in a complex known as trypanosome lytic factor-1 (TLF-1) that contains APOL1, APOA1, and HDL3. The HPR subunit of the complex binds hemoglobin with an unknown stoichiometry (Shiflett et al. 2005, Nielsen et al. 2006, Widener et al. 2007, Harrington et al. 2009).