The x gene of immunoglobulin is effectively transcribed in extracts from homologous, i. e. lymphoid cells; the transcription is weak and incorrect in a heterologous system. x gene fragments with deletions in the 5'-region of the gene and with transpositions of the enhancer have been constructed. If the enhancer is removed, the transcription becomes weaker, although it remains tissue-specific. The transcription is weakened abruptly by removing a region preceding the CAT-box, in which the conservative TNATTTGCAT sequence is located. The transcription is tissue-specific due to a protein factor which is contained in the lymphoid cells. When this factor is repeatedly purified and added to an extract from HeLa cells, the transcription becomes much more effective and its initiation point is corrected. The transcription factor exerts a distinct action on templates with the native promoter, but the action decreases abruptly if the dc region of the gene is removed. The results indicate that the protein factor(s), which recognizes the DNA region preceding the CAT box, is necessary for the effective, tissue-specific and correct transcription of the x gene.