Analysis of μ expression by Northern blot. As described in the text, multiple independent replacements were isolated for each vector, and the concentration of IgM in culture supernatant of these cell lines was measured by ELISA. RNA from the cell lines with the highest and lowest IgM concentration was analyzed by Northern blot and probed with segments of the μ and κ genes. The intensity of the bands was quantified by phosphorimager, and the μ/κ ratio, normalized to the value for Sp6 hybridoma, is indicated below each lane. The IgM concentration for each cell line, also normalized to the value for Sp6, is listed below each lane.