By mutagenesis and selection, two independent Ia- mutant cell lines derived from the Ia+ human B cell line Raji were isolated. One mutant failed to express detectable surface HLA-DR and HLA-DQ molecules, while the other expressed very low levels of these molecules. mRNA encoding Ia molecules was correspondingly absent or reduced in these cell lines. In contrast, levels of class I major histocompatibility complex (MHC) mRNA and protein were normal in these cells. The class II MHC genes of the mutant cells were grossly intact, suggesting that the defective expression of these genes was due to an absent or abnormal trans-acting regulatory factor. In vitro nuclear run-on transcription assays and measurement of class II MHC mRNA stability suggested that the specific defect in class II MHC gene expression in both mutant cell lines was at the level of transcription initiation.

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