Autoantibody levels in sera of factor V Leiden (FVL) and C57/B6 mice. Antiphospholipid syndrome (APS) was induced in female mice by immunization with β2-glycoprotein I (β2-GPI), whereas controls were immunized with adjuvant (complete Freund’s adjuvant, CFA) alone. Anti-cardiolipin β2-GPI-dependent (anti-CL(β2-GPI)) antibodies were measured in APS (FVL+/+-APS, n = 10), control FVL+/+ C57BL/6 background (FVL+/+-control, n = 11), FVL heterozygous APS (FVLQ/+-APS, n = 8), FVL homozygous APS (FVLQ/Q-APS n = 7),and FVL heterozygous control (FVL-control, n = 15) mice. Titers were measured at (A) 1 month post-immunization (1mo PI) and (B) 5 months post-immunization (5mo PI). The levels of antibody represent individual and mean ± SE absorbance values for ELISA. The FVL mice, and especially the FVLQ/Q-APS group, developed significantly higher levels of anti-CL(β2-GPI) antibodies at 5mo PI compared with the control background mice, in which the levels of antibodies dropped over time.