The probe for (TTAGG)n telomeric DNA was generated by polymerase chain reaction (PCR) with the oligonucleotides (TTAGG)5 and (CCTAA)5 as primers as described . Labeling was achieved by additional PCR cycles replacing dTTP (=desoxythymidintriphosphat) by Tetramethyl-Rhodamine-5-dUTP (Tamra-5-dUTP). For detection of 18S rDNA a fragment of human 18S rDNA cloned in pHr13 165  was applied. The rDNA-probe was labeled with Alexa 488-5-dUTP by nick-translation according standard protocols .