1. School of Biological Sciences, University of Manchester, 2.205 Stopford Building, Oxford Road, Manchester M13 9PT, UK
2. Present address: Instituto de Parasitología y Biomedicina 'López-Neyra', CSIC, C/ Ventanilla 11, 18001 Granada, Spain.

Correspondence to: Miguel Navarro¹ Correspondence and requests for materials should be addressed to M.N. (e-mail: Email: miguel.navarro@ipb.csic.es).

In the mammalian host, African trypanosomes generate consecutive waves of parasitaemia by changing their antigenic coat. Because this coat consists of a single type of variant surface glycoprotein (VSG), the question arises of how a trypanosome accomplishes the transcription of only one of a multi-allelic family of VSG expression site loci to display a single VSG type on the surface at any one time¹. No major differences have been detected between the single active expression site and the cohort of inactive expression sites². Here we identify an extranucleolar body containing RNA polymerase I (pol I) that is transcriptionally active and present only in the bloodstream form of the parasite. Visualization of the active expression site locus by tagging with green fluorescent protein³ shows that it is specifically located at this unique pol I transcriptional factory. The presence of this transcriptional body in postmitotic nuclei and its stability in the nucleus after DNA digestion provide evidence for a coherent structure. We propose that the recruitment of a single expression site and the concomitant exclusion of inactive loci from a discrete transcriptional body define the mechanism responsible for VSG mono-allelic expression.