Immunoglobulin(Ig) heavy chain gene-transcribing cell diffrentiation was investigated in the thymus of BALB/c mice by the Realtime PCR for membrane-bound form IgM(IgM_m). The ratio of IgM_m to total IgM(IgM_t) was approximately 1 from fetus period to 14 days after the birth, and IgM transcriptions were almost processed to membrane-bound form. Then IgM_m transcriptions decreased and secretory form IgM(IgM_s), the difference between IgM_t and IgM_m, started being detected. It was concluded that thymus Ig gene-transcribing cells underwent the similar differential processes in which B cells developed from immature type having IgM_m to mature type secreting IgM_s, in secondary lymphoid tissues
