DNA methylation analysis. Distal and proximal CpG sites of the mouse Scgb3a1 gene promoter were subjected to direct DNA sequencing after bisulfite treatment and PCR amplification of DNAs isolated from mtCC, NIH3T3 and mouse lungs. Sequence containing CpG site is shown at the top, and CpG site is boxed. Cytosine residues are underlined. If these cytosine residues are unmethylated, they appear as thymine after bisulfite treatment (upper panel; mtCC unmethylated, shown by a black arrow). The cytosine residue that is methylated stays as cytosine even after bisulfite treatment (middle panel; NIH3T3 methylated, shown by a red arrow). Lung DNA shows both T and C residues due to a mixture of cell types of methylated and unmethylated (bottom panel; Lung mixed, shown by a black and red arrow).