Identification of the messenger (m) RNA coding for the constant fragment (c mu) of the heavy chain with cloned DNA in single cells by in situ hybridization.

Cells from a normal donor, a B-CLL and a T-ALL were labeled with an FITC conjugated rabbit antihuman IgM and hybridized with a rhodamine-conjugated cloned DNA coding for the c mu segment. The cells were measured simultaneously by immunofluorimetry for their surface content of IgM and their amount of m-RNA hybridizing with the cloned DNA. Thus it was possible to compare the transcription of the gene coding for the immunoglobulin mu chain and the expression of IgM in individual cells. It could be shown that besides the immunoglobulin mu containing B-cells, which were expected to contain the respective m-RNA, cells were found which did not express IgM but which were still positive for the m-RNA.