Novel approaches to the structural and functional analysis of mammalian chromosomes would be possible if the gross structure of the chromosomes in living cells could be engineered. Controlled modifications can be engineered by conventional targeting techniques based on homologous recombination. Large but uncontrolled modifications can be made by the integration of cloned human telomeric DNA. We describe here the combined use of gene targeting and telomere–mediated chromosome breakage to generate a defined truncation of a human chromosome. Telomeric DNA was targeted to the 6–16 gene on the short arm of chromosome 1 in a human cell line. Molecular and cytogenetic analyses showed that, of eight targeted clones that were isolated, one clone had the predicted truncation of chromosome 1.