BL22 (RFB4(dsFv)-PE38) is a recombinant Pseudomonas exotoxin-based immunotoxin under development by the National Cancer Institute for the treatment of B-cell malignancies. It is composed of the disulfide-stabilized Fv portion of the anti-CD22 antibody RFB4 genetically fused to a truncated form of Pseudomonas exotoxin A. It has entered phase I trials for the treatment of B-cell lymphoma.