In vitro ELISA binding of anti-rFRβ Fabs and affinity-matured clone m909. (a) The parental Fab clone and light-chain shuffled clones were tested on ELISA wells coated with rFRβ. All Fabs were purified from periplasm of Escherichia coli and tested from 1,500 to 0.0192 nM. m909 was the clone featured in the study. LC38 and LC41 are two other affinity-matured clones. (b) m909 IgG was tested for binding to rFRβ on ELISA. The highest IgG1 concentration tested was 1,000 nM and was serially diluted at 1:10. m909 IgG showed binding even at 1 fM. ELISA, enzyme-linked immunosorbent assay; Fab, antigen-binding fragment; OD, optical density; rFRβ, recombinant folate receptor beta.