Subsets of uni-SLSs. SLSs located between unidirectionally transcribed CDSs have been subdivided into three categories, relatively to their distance from flanking CDSs as indicated at the bottom. The position of the dotted lines across bars denotes the averaged amount of intergenic space occupied by the three categories of SLSs analyzed. The uni-IGS minimum size was raised to 129 bp, since shorter IGSs cannot be assigned to any of the three categories. SLSs were selected as in Fig. 6.