Terminal telomere repeats are actually short in telomerase-negative immortal human cells.

Telomerase-negative immortal human cells maintained telomere length by a mechanism called alternative lengthening of telomeres (ALT mechanism). These cells (ALT cells) have two prominent characteristics of long telomere repeats at each chromosome end revealed by Southern blotting (terminal restriction fragments: TRF) and the presence extrachromosomal telomere repeat (ECTR) DNA. We report here that the TRF length of ALT cells revealed by the conventional unidirectional (UD) current or pulse-field (PF) current electrophoresis appeared to be over estimated.