Identification of YR Rule. (A) Dinucleotide sequences at the -1/+1 position relative to Arabidopsis TSS, determined by information of the fl-cDNAs, were counted. As shown, most of the TSS have (C/T)(A/G), and this YR Rule applies to 77% of the analyzed TSSs. (B) Frequency of dinucleotide sequences fitting with YR Rule was scanned from -5 to +5 of Arabidopsis and rice TSS. Position of the downstream site of the dimer is shown. For example, the -1/+1 position is indicated as "1". Theoretically frequency of YR in non-biased sequence is 0.25.