The year 2001 witnessed the sequencing of 90% of the euchromatic region in the 
human genome but the ultimate goal to delineate the positions of all genes is 
yet to be achieved. Fluorescence In Situ Hybridization (FISH) is one of the 
methods for localizing genes on chromosomes. In the present study, diagnostic 
utility of single-, dual-, and multicolor FISH was evaluated for prenatal 
diagnosis, cancer genetics, and screening of various congenital anomalies (sex 
chromosomal and autosomal). Centromeric probes for chromosomes X and Y were used 
for screening minor aneuploid cell lines (XXY, XO, and XXX) in the cases of 
primary amenorrhea and suspected Klinefelter syndrome. The cases with ambiguous 
genitalia were analyzed using a probe specific for the sex-determining region 
(SRY). Suspected cases of Down syndrome were subjected to FISH using probe 
specific for chromosome 21. FISH was also used to study gene alterations in 
retinoblastoma and myeloid leukemias. Prenatal diagnosis was done to screen for 
aneuploidies of chromosomes 13, 18, 21, X, and Y using FISH on uncultured cells 
from amniotic fluid and chorionic villi sampling. The screening for common 
aneuploidies was extended to abortuses from spontaneous abortions. Using FISH, 
low-level mosaicism could be identified in some cases of primary amenorrhea and 
suspected Klinefelter syndrome. Submicroscopic gene rearrangements could be 
detected using FISH in cases of ambiguous genitalia and cancers. Further 
interphase FISH could provide results within 24 hours. To conclude, FISH adds to 
the diagnostic utility of routine cytogenetics and its use on interphase nuclei 
overcomes the difficulty of conventional cytogenetics, thereby reducing the time 
between sampling and diagnosis to 24 hr.