The monoclonal antibody C12.38 specifically recognizes AID. (A) Western blots of lysates from LMH cells transfected with a vector containing the coding sequence of AID and the SV40 T-antigen (LMH-AID/T-antigen) or mock vector (LMH-mock) were probed with mAb recognizing AID (clone C12.38), T-antigen, or β-actin as a loading control. Antibodies against AID and T-antigen both label a 47-kDa AID/T-antigen fusion protein in AID/T-antigen-transfected LMH cells, while mock-transfected cells are not reactive. (B) Cytospins of LMH cells transfected with AID/T-antigen, but not with mock vector stain with anti-AID. A nuclear staining detected in some LMH cells was probably nonspecific, since it was occasionally observed in tissues unlikely to express AID, such as epithelium. (C) Anti-AID mAb detects a 24-kDa band in Western blots from lysates of the human Burkitt lymphoma cell lines P3HR1and Raji, whereas Nalm6, Reh, HSB-2, and Jurkat lysates yield no signal. β-Actin probing was used as a loading control. (D) In cytospin preparations, P3HR1 and Raji cells stain with anti-AID. No labeling is seen with Nalm6, Reh, HSB-2, and Jurkat. (E) RT-PCR reveals AID expression in P3HR1 and Raji cells, but not in Nalm6, Reh, HSB-2, or Jurkat. All cell populations are strongly positive for the housekeeping gene GAPDH. Magnification: 40×/0.75 NA objective (B,D).