Map of sequence elements in the human rDNA repeat unit and identification of the BLM association domains. A. The human rDNA repeat unit (U13369) sequence features and the location of the region isolated with a polyclonal BLM antibody. The orange arrows indicate sites of primer sets. The dotted lines indicate the direction of DNA replication origins . B. ChIP mapping of BLM association with the rDNA repeat and mapping of BLM domains required for rDNA association. Equivalent amounts of DNA recovered from the induced cell lines were amplified with Taq DNA polymerase and one of eight primer sets, denatured and transferred to a nylon membrane. The PCR products were hybridized with the radiolabeled forward primer in each set and quantitated using a phosphorimager. Hybridization units were calculated relative to the cell line expressing EGF alone. C. A simple model for the BLM-binding region of the rDNA repeat.