text: Flow-FISH was first published in 1998 by Rufer et al. as a modification of another technique for analyzing telomere length, Q-FISH, that employs peptide nucleic acid probes of a 3-CCCTAACCCTAACCCTAA-5 sequence labeled with a fluorescin fluorophore to stain telomeric repeats on prepared metaphase spreads of cells that have been treated with colcemid, hypotonic shock, and fixation to slides via methanol/acetic acid treatment Images of the resultant fluorescent spots could then be analyzed via a specialized computer program to yield quantitative fluorescence values that can then be used to estimate actual telomere length.  The fluorescence yielded by probe staining is considered to be quantitative because PNA binds preferentially to DNA at low ionic salt concentrations and in the presence of formamide, thus the DNA duplex may not reform once it has been melted and annealed to PNA probe, allowing the probe to saturate its target repeat sequence (as it is not displaced from the target DNA by competing anti sense DNA on the complementary strand), thus yielding a reliable and quantifiable readout of the frequency of PNA probe target at a given chromosomal site after washing away of unbound probe.