Our previous report demonstrated a good correlation between high telomerase activity of cancer tissues and a poor prognosis of patients with colorectal cancers, except for several cases. To elucidate the additional factors that contribute to patient prognosis, the correlation among the expression levels of telomere binding proteins (TBP), the lengths of telomeres, the lengths of telomere 3¢-overhang (3¢-OH) and telomerase activity in 106 paired colorectal cancer and corresponding noncancerous mucosa (NCM) specimens were examined. The expression levels of eight TBP genes (TRF1, TRF2, TIN2, TANK1, TANK2, POT1, RAP1 and TPP1) were analyzed. Among the 106 cases, 35 cases had shortened telomeres (<7 kb), 15 had shortened 3¢-OH (3¢-OH length ratio of cancer/NCM <0.5) and 88 were classified as telomerase-activated cancers (activity ratio of cancer/NCM >2). Comparison between NCM and cancer in each case showed that all TBP except for POT1 were downregulated in cancers. A survival analysis using a Cox proportional hazard model showed that the survival rate of the telomerase-activated cases with shortened 3¢-OH and that of telomerase-inactivated cases were significantly better than that of telomerase-activated cases without 3¢-OH shortening, that is, restored or maintained 3¢-OH (P = 0.018). In the telomeraseactivated cancers, the length of 3¢-OH was significantly correlated with the expression levels of POT1. Elongation of telomeric overhang by telomerase, which might be regulated by POT1, may contribute to the increase of malignant potential in colorectal cancers. (Cancer Sci 2011; 102: 330-335) T he ends of chromosomes, named telomeres, are characterized by guanine-rich tandem hexamers (5¢-TTAGGG-3¢) with an average length of 5-15 kb and they serve as protective caps.(1,2) During DNA replication in cell division, telomeres gradually shorten at a rate of 50-200 bp as a result of the incomplete replication of linear chromosomes, the so-called 'end-replication problem'.(3) The telomeres terminate in a 3¢-single-stranded overhang of 12-300 bp, that is, ''telomere 3¢-overhang (3¢-OH)'' of the G-rich strand, which can be extended by telomerase in the S phase, followed by fill-in of the C strand in the late S phase.(4) Telomerase is a ribonucleoprotein that catalyzes de novo synthesis and elongation of telomeric repeats at chromosomal ends using an intrinsic RNA template in eukaryotic cells with an extended lifespan. The telomere overhang folds back into the D-loop of the duplex telomeric DNA to form a protective ''T-loop'' that prevents degradation by exonucleases or processing as damaged DNA.(5) This loop hides the 3¢ end also from telomerase. Furthermore, the telomeric repeats are bound to a large protein complex called ''shelterin'',which involves several telomere binding proteins (TBP). This complex is proposed to regulate telomere length, telomere protection and other cell functions such as proliferative activity.(5) When the telomere length shortens to a limited length, cells fall into senescence due to cleavage of ...