Domain observed N-terminal to the pPIWI_RE domain module. PIWI/Argonaute modules are typically N-terminally fused to the PTND1 and PTND2 domains [1], pPIWI_RE_X may represent an ancestral version of these domains or represent a N-terminal innovation predating the emergence of PTND1 and PTND2 [2].

1: New perspectives on the diversification of the RNA interference system: insights from comparative genomics and small RNA sequencing. Burroughs AM, Ando Y, Aravind L; Wiley Interdiscip Rev RNA. 2014;5:141-181. PMID:24311560

2: Two novel PIWI families: roles in inter-genomic conflicts in bacteria and Mediator-dependent modulation of transcription in eukaryotes. Burroughs AM, Iyer LM, Aravind L; Biol Direct. 2013;8:13. PMID:23758928