L-Histidine decarboxylase (HDC) catalyzes the formation of histamine from L-histidine, and in hematopoietic cell lineages the gene is expressed only in mast cells and basophils. We attempted here to discover how HDC gene expression is restricted in these cells. In the cultured cell lines tested, only the mast cells and basophils strongly transcribed the HDC gene. However, in transient transfection analysis, the reporter constructs with the HDC promoter were active not only in expressing cells but also in nonexpressing cells. Detailed analyses of the HDC promoter region revealed that the GC box is essential for transactivation. Also, the promoter region of the HDC gene proved to be sensitive to DNase I and restriction endonucleases exclusively in HDC-expressing cells, suggesting that the promoter region is readily accessible to trans-acting factor(s). Furthermore, the promoter region in HDC-expressing cell lines was found to be selectively unmethylated. The correlation between HDC expression and hypomethylation was also found in primary human mast cells. Methylation of the HDC promoter in vitro reduced the luciferase reporter activity in transient expression analysis, suggesting that methylation of the promoter region is functionally important for HDC gene expression. These results imply that alteration of DNA methylation is one of the mechanisms regulating cell-specific expression of the HDC gene.