The introns separating the variable and constant regions of active immunoglobulin genes contain tissue-specific transcriptional enhancer elements, DNA segments which act in cis in an orientation- and distance-independent (up to a few kilobases (kb)) manner to enhance transcription initiation at adjacent promoters. The immunoglobulin heavy-chain enhancer is active only in lymphoid cells: in transfection assays it is capable of controlling in cis transcription from the simian virus 40 (SV40) T-antigen, rabbit beta-globin and immunoglobulin gene promoters up to at least 2 kb away. Genetic deletion analysis suggests that a region of as few as 140 base pairs (bp) is sufficient for the enhancement effect. These functional characteristics and DNA sequences are conserved between mouse and man. However, it is not known whether tissue-specific proteins bind to the enhancer. Proteins that interact with DNA at specific sequences can prevent or enhance the reactions of individual guanines or adenines with dimethyl sulphate (DMS), and this property has been used to display the DNA contacts of various regulatory proteins. Here we apply this DMS strategy in experiments involving single-copy genes within intact mammalian nuclei using genomic sequencing.