DNA oligomers with cytosine bases (C-pyr, C-tpp, and C-pnx) with bi-and tricyclic ring structures were synthesized. Among the modified bases, C-pyr was found to enhance the duplex stability retaining the base recognition ability. C-tpp was effective in neither hybridization nor base recognition. C-pnx could form more strongly a matched base pair with guanine but lost more significantly base recognition than C-pyr.