For a decade, mystery has surrounded the mechanisms by which piRNA biogenesis yields distinct size classes of small RNAs within individual PIWI proteins. In this issue of Cell, two studies shed light on this process, identifying conserved PARN-family exonucleases that trim piRNAs to their mature size in silkworms and C. elegans.

Decay of pre-existing ribonucleic acid was studied in Escherichia coli cells subjected to high temperature or to starvation for nitrogen, phosphate, amino acids, or a carbon source. In these studies, a series of mutants affected in ribonuclease I (RNase I, EC 3.1.4.22) polynucleotide phosphorylase (EC 2.7.7.8) or ribonuclease II (RNase II, EC 3.1.4.23) were used. Degradation of total RNA and th...

We introduce and investigate an operation with strings suggested by DNA processing (by means of exonucleases): the operation of cutting strings of equal length from the beginning and the end of a string. A related operation is that of cutting a square of a string from the preex of a string. The closure properties of families in the Chomsky hierarchy are investigated (and, with one exception, fo...