The mechanism for the steroidogenic tissue or cell-specific expression of SF-1 has not been well clarified. We examined whether the methylation status of a large CpG island in the first intron of mouse SF-1 gene is associated with the expression level of SF-1 in cultured cells and in tissues. The island consists of three small islands (ICI-1, ICI-2, and ICI-3). In cultured adrenocortical Y-1 cells and in Leydig tumor cells, I-10, that both express high levels of SF-1, the upstream region of ICI-2, ICI-2-1, was clearly hypomethylated compared to cultured mouse bone marrow cells that do not express SF-1. However, this methylation status was not clearly associated with the tissue-specific expression of SF-1, in either adult or during development. These results suggest that methylation of ICI-2-1of SF-1 may partly determine the level of SF-1 expression at the cellular level, but may not be essential at the tissue level.