*2.2.4 Quantitative fluorescence in situ hybridization (Q-FISH)*

Quantitative fluorescence *in situ* hybridization (Q-FISH) of telomeric repeats is performed by evaluating metaphase or interphase nuclei after hybridization or labeling with a fluorescent (CCCTAA) three probe [83]. The substrate for Q-FISH, in contrast to the TRF and PCR-based tests, is cells. When used for Q-FISH, cells can be fresh, frozen, formalin-fixed, paraffin-embedded, or permeabilized. The Q-FISH approach can be used for telomeres of any size, not just average or little ones. Additionally, only by this method of evaluation will "telomere-free" endings be identified [84, 85]. For understanding how telomere length varies between chromosomes and how frequently telomere-free ends are associated with chromosomal instability, metaphase Q-FISH experiments have proved essential [86]. It is believed to be particularly helpful for figuring out the telomere length in unusual cells [87]. To correct some of its shortcomings, the use of interphase cells rather than metaphase chromosomes in these modifications is one of the Q-FISH advancements [88, 89].

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