Alternative lengthening of telomeres in human cells

Activation of a telomere maintenance mechanism appears to be essential for immortalization. In most human tumors and tumor cell lines, the telomere ma intenance mechanism involves the activity of telomerase, a reverse transcri ptase holoenzyme that synthesizes telomeric repeat DNA, In some cases, telo mere maintenance occurs in the absence of telomerase activity by a mechanis m referred to as alternative lengthening of telomeres (ALT). The developmen t of telomere-targeted anticancer therapies will be facilitated by an under standing of the molecular mechanisms of ALT and of the means whereby ALT is repressed in normal cells. (C) 2001 by Radiation Research Society.