biostudies-literatureUnknownChowdhury TMedical Research CouncilWellcome TrustThe mouse PIWI-interacting RNA (piRNA) pathway provides sustained anti-transposon immunity to the developing male germline by directing transposon DNA methylation^1-3. The first step in this process is the recruitment of SPOCD1 to young LINE1 loci⁴. Thereafter, piRNA-mediated tethering of the PIWI protein MIWI2 (also known as PIWIL4) to the nascent transposon transcript recruits the DNA methylation machinery^5,6. The piRNA pathway needs to methylate all active transposon copies but how this is achieved remains unknown. Here we show that nuclear piRNA and de novo methylation factors are all euchromatic, exposing constitutive heterochromatin as a genomic blind spot for the piRNA pathway. We discover a 'nowhere-to-hide' mechanism that enables piRNA pathway-mediaNaturehttps://www.ebi.ac.uk/biostudies/studies/S-EPMC7618654biostudies-literatureA nowhere-to-hide mechanism ensures complete piRNA-directed DNA methylation.PMC7618654MC_UU_00035/7217120/Z/19/Z226791092076200898217120225237108504203149O'Carroll DSpanos CZou JKelly DFieler HZoch ABickmore WACook AGBoyle SMirandela MDChowdhury TXiang XfalseA nowhere-to-hide mechanism ensures complete piRNA-directed DNA methylation.The mouse PIWI-interacting RNA (piRNA) pathway provides sustained anti-transposon immunity to the developing male germline by directing transposon DNA methylation^1-3. The first step in this process is the recruitment of SPOCD1 to young LINE1 loci⁴. Thereafter, piRNA-mediated tethering of the PIWI protein MIWI2 (also known as PIWIL4) to the nascent transposon transcript recruits the DNA methylation machinery^5,6. The piRNA pathway needs to methylate all active transposon copies but how this is achieved remains unknown. Here we show that nuclear piRNA and de novo methylation factors are all euchromatic, exposing constitutive heterochromatin as a genomic blind spot for the piRNA pathway. We discover a 'nowhere-to-hide' mechanism that enables piRNA pathway-media2026-01-01T00:00:00Z2026 Jan2026-07-16T03:11:49.81Z2026-07-09T10:30:53.186ZS-EPMC76186544153545710.1038/s41586-025-09940-w