Constitutive and tissue-specific protein factors which bind to transcriptional regulatory elements of Ig genes (promoter and enhancer) are described. The factors were identified and isolated by an improved assay for protein-DNA binding. Genes encoding factors which positively regulate transcription can be isolated and employed to enhance transcription of Ig genes. In particular, NF-kB, the gene encoding NF-kB, IkB and the gene encoding IkB and uses therefor.