AID protein expression and subcellular localization in normal B lymphocytes. (A) AID mRNA expression in purified normal B-cell subpopulations (5 samples each of N [naive], CB [centroblasts], CC [centrocytes], M [memory]). AID was represented by 2 probe sets in the Affymetrix U133Plus GeneChip array. BCL6, BCL7A, and the proliferation-associated gene Ki67 are shown along with BCL2 and TOSO as markers for the identity of the purified subpopulations. (B) Western blot analysis of AID and β-actin expression in whole cell extracts obtained from naive and CB populations. (C) Western blot analysis of AID expression in nuclear (nu) and cytoplasmic (cy) extracts from tonsillar GC B cells. The purity of the fractions was monitored using anti-BCL6 and anti-α-tubulin Abs, respectively.