Flow-FISH (fluorescence in-situ hybridization) is a cytogenetic technique to quantify the copy number of RNA or specific repetitive elements in genomic DNA of whole cell populations via the combination of flow cytometry with cytogenetic fluorescent in situ hybridization staining protocols.
Flow-FISH is most commonly used to quantify the length of telomeres, which are stretches of repetitious DNA (hexameric TTAGGG repeats) at the distal ends of chromosomes in human white blood cells, and a semi-automated method for doing so was published in Nature Protocols.  Telomere length in white blood cells has been a subject of interest because telomere length in these cell types (and also of other somatic tissues) declines gradually over the human lifespan, resulting in cell senescence, apoptosis, or transformation.  This decline has been shown to be a surrogate marker for the concomitant decline in the telomere length of the hematopoietic stem cell pool, with the granulocyte lineage giving the best indication, presumably due to the absence of a long lived memory subtype and comparatively rapid turnover of these cells.
Flow-FISH is also suitable for the concomitant detection of RNA and protein. This allows for the identification of cells that not only express a gene, but also translate it into protein. This type of Flow-FISH has been used to study latent infection of viruses such as HIV-1 and EBV, but also to track single cell gene expression and translation into protein.