Active genes for variant-specific surface glycoproteins (VSGs) reside in telomeric expression sites and may be replaced by other VSG genes via telomere conversions. The availability of a complete map of expression site 221 in variant 221a made it possible to determine the boundaries of such conversions and the sequences that are involved. We have analysed five trypanosome populations that arose from variant 221a through replacement of the 221 gene by another VSG gene. In each of these relapsed populations the telomere conversion ends at a different position in the expression site. In the relapsed population, 221aR3, the boundary was found in the coding region of an expression-site-associated gene (ESAG). This ESAG-2 codes for a potential 368-aa protein of unknown function; it contains a N-terminal signal peptide for mediating transfer to the endoplasmic reticulum and six potential N-glycosylation sites. It shares these structural features with the ESAG-1 protein encoded in the same expression site. ESAG-2 is a member of a large gene family which includes non-functional genes. In 221aR3, the partial conversion of ESAG-2 by an ESAG-2-like sequence has disrupted the open reading frame. The two ESAG-2 sequences are similar (92% identity) suggesting that sequence homology between telomeres provides the opportunity for gene conversion.