The Piwi box (or Sting domain) is an about 40 to 80 amino-acid conserved region, which was first identified in the C-terminal part of Drosophila Piwi and Sting proteins, and of related proteins from human, Caenorhabditis elegans, Arabidopsis thaliana, fission yeast and Dictyostelium discoideum [1,2]. It was later recognized that this conserved region is part of a larger 300-amino-acid domain, called Piwi, which is also found in some prokaryotic proteins of unknown function [3]. The Piwi domain containing proteins are often involved in stem-cell maintenance and/or post-transcriptional gene silencing (PTGS) through RNA interference (RNAi), the mechanism through which double-stranded RNAs (dsRNAs) silence cognate genes [3]. Neither the structure nor the function of the Piwi domain are known.

Some proteins known to contain a Piwi domain are listed below:

• Drosophila Piwi protein. It is a nuclear protein that acts a key regulator of stem cell division. Its somatic expression modulates the number of germline stem cells and the rate of their division, while its germline expression also contributes to promoting stem cell division in a cell- autonomous manner. The Piwi domain is essential for its function [4].
• Drosophila Sting protein. It could repress general or specific transcript processing.
• Human Hiwi (for human Piwi) protein. It might play a role in maintenance of stem cell quiescence or downregulation of stem cell or progenitor cell cycling.
• Caenorhabditis elegans Piwi-related proteins 1 and 2 (prg-1 and prg-2). They are also required for germ-line self-renewal.
• Caenorhabditis elegans RNA Interference Deficient 1 (rde-1). It is required for RNAi and transposon silencing.
• Arabidopsis thaliana Argonaute (Ago) protein. It is involved in RNAi as well as in leaves and floral organs development and axillary meristems formation.
• Arabidopsis thaliana Zwille (Zll) or Pinhead (Pnh). It is a developmental protein required for reliable formation of primary and axillary shoot apical meristems.
• Eukaryotic translation initiation factor 2C (eIF2C). It plays an important role in the eukaryotic peptide chain initiation process.

The profile we have developed covers the entire Piwi domain.