The DNaseI hypersensitive site at −2 kb has B cell–specific enhancer activity. (A) Map of the mouse Cd19 locus on chromosome 7. (Top panel) Position of EcoRI sites and probe 1 used in Southern blotting analysis. (Middle panel) Position of introns, exons, transcription start of Cd19, and neighboring genes and conservation between different species (http://genome.ucsc.edu/). indicates the position of DHS corresponding to the promoter and the enhancer. (B) DNaseI hypersensitive site mapping assays demonstrating strong hypersensitive sites at −2 kb and the promoter. Genomic DNA from DNaseI-treated cells was digested with EcoRI and subjected to Southern blot analysis using probe 1. Cells used were mouse embryonic fibroblast (fibroblasts), bone marrow-derived macrophages (Mø), pro-B cells (pro-B), CD19⁺ splenocytes (splenic B), Thy1⁺ thymocytes (thymic T), and Thy1⁺ splenocytes (splenic T). (C) Transient transfection assays in different mouse cell lines. A mouse Cd19 promoter reporter construct (−7 to −200 bp; □) or a construct containing the −2 kb DHS (−1832 to −2096 bp) combined with the Cd19 promoter (■) was transiently transfected into various cell lines. The relative activity was determined as the luciferase activity of each construct over control vector pXPG. Data represent the mean of 2 to 4 experiments performed in triplicate.