Replication forks progress toward the telomere from origins in the subtelomere of chromosome 10q in three cell types. SMARD analysis of the 10q telomere segment in ES lines H9 and H1, primary lung fibroblast, IMR-90 cells, and primary microvascular endothelial MECs indicated that the four cell lines replicate this segment using very similar programs. In these lines, the 10q telomere is replicated primarily by forks progressing from the subtelomere to the telomere. This is reflected in the replication profile histograms, which show a progressive decrease in the percentage of IdU incorporation from 5′ to 3′ across the segment, which is indicative of replication progressing through the segment primarily in one direction from an external origin. These distal subtelomeric origins are located predominantly >200 kb from the telomere in all four lines. A region of mixed staining (blue-green or blue-red) is seen in the right half of many molecules resulting from nonspecific hybridization of the FISH probes. The yellow arrows indicate the direction of replication fork progression, and the vertical orange and blue lines demarcate the boundaries of sequences where fish probes bind, as described in the legend to Fig. 2.