Enhancer sequences are regulatory regions that greatly increase transcription of certain eukaryotic genes. An immunoglobulin heavy-chain variable gene segment is moved from a region lacking enhancer activity to a position adjacent to the known heavy-chain enhancer early in B-cell maturation. In lymphoid cells, the heavy-chain and SV40 enhancers bind a common factor essential for enhancer function. In contrast, fibroblast cells contain a functionally distinct factor that is used by the SV40 but not by the heavy-chain enhancer. The existence of different factors in these cells may explain the previously described lymphoid cell specificity of the heavy-chain enhancer.