We recently generated histone deacetylase2 (HDAC2)-deficient DT40 mutants HDAC2(-/-) and analyzed their characteristics at various intervals during cultivation. In this article we reviewed our studies on several characteristics of HDAC2(-/-) mutants. In six independent clones (cl.2-1 to cl.2-6) of HDAC2(-/-) mutants, proteins and mRNAs of IgM H- and L-chains were dramatically increased at the early stage of cultivation, and thereafter decreased in almost similar changing pattern and at the later stage reached comparable levels as in DT40 cells. By contrast, mRNAs of various transcription factors and chromatin-modifying enzymes showed distinct changing patterns in these six HDAC2(-/-) clones during cultivation. In clone cl.2-1, mRNAs of Pax5, Aiolos and EBF1 were almost completely decreased at the early stage and remained unchanged during cultivation, but that of OBF1 was dramatically decreased until the later stage. In clones cl.2-2, cl.2-3, cl.2-4 and cl.2-5, mRNAs of Pax5, Aiolos and EBF1 were almost completely decreased at the early stage, and thereafter mRNAs of Pax5 and Aiolos were increased until the later stage but that of EBF1 remained unchanged during cultivation. In clone cl.2-6, mRNAs of Pax5, Aiolos and EBF1 were almost completely decreased at the early stage and thereafter dramatically increased during cultivation. These findings suggested that three distinct ways of gene expressions of IgM H- and L-chains exist at the later stage in six individual HDAC2(-/-) clones; i.e., clone cl.2-1 seems to be the OBF1-dependent type, clones cl.2-2, cl.2-3, cl.2-4 and cl.2-5 seem to be the Pax5- and Aiolos-dependent type, and clone cl.2-6 seems to be the Pax5-, Aiolos- and EBF1-dependent type. These results revealed that individual clones of HDAC2(-/-) mutants have the ability to gain new cell function to exclude artificially accumulated IgM H- and L-chains depending on alterations in gene expressions of Pax5, Aiolos, EBF1, OBF1, etc. through various generations during continuous cultivation.