Recent data indicates that controlled in vivo synthesis of telomeric DNA is a 'menage a trois', in which sister chromatid termini paired by telomere end-binding protein constitute a core substrate for a telomerase dimer. Such an arrangement could serve to fine tune synthesis of telomeric DNA and might ensure similar telomerase processivity for paired sister chromatid termini, possibly to secure the maintenance of individual telomere lengths in daughter cells. It also suggests that the bona fide end-capping protein, which is likely to regulate telomerase by restricting access to chromosome termini, has escaped detection in man.