was very small even for libraries as long as 100 nt. The prevalence of 8–17 in our study is consistent with these ﬁnd- ings, and suggests that 80 random nucleotides may still represent a sub-optimal length for ﬁnding new, complex, and more efﬁcient RNA-cleaving deoxyribozymes. Of course, the beneﬁts of increasing the length of the random-sequence domain may be irrelevant, if the limiting factor is actually the relatively permissive reaction time ( 5 s) that can be imposed as a selection constraint by conventional manual quenching techniques, and/or the small fraction of sequence space that can be searched in any single experiment. These constraints could potentially be overcome as automated SELEX (60–63) and continuous evolution (64) strategies become more widely applicable. In the meantime, case studies such as this one will continue to provide signiﬁcant clues to the global distribution of deoxyribozymes in sequence space and underscore the current limitations in accessing them. 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