In situ hybridization allows the detection of specific nucleic acid sequences in morphologically intact cells and chromosomes. Presently, fluorescence in situ hybridization (FISH) has reached a high detection sensitivity (defined as the smallest DNA target detectable, high DNA resolution (defined as the smallest distance in kilobasepairs between two DNA targets that can be resolved microscopically and a high multiplicity (defined as the number of different probes that can be identified simultaneously. The current status of FISH methodology:*several direct and indirect nucleic acid modifications*sensitivity: unique DNA sequences 1-5 kb*DNA resolution: metaphase ~ > 1-3 Mbp (light microscopy) interphase ~50 kbp DNA halos ~ 1 kbp*multiplicity: at least 12Progress in in situ hybridization and related technology has caused molecular cytogenetics to become an established. In particular the ability of ISH techniques to detect chromosomes and/or chromosome parts in interphase nuclei (interphase cytogenetics) has significantly contributed to this acceptance of ISH, since this technique provides statistically reliable means to study the genetic composition of cells that can not, or only by complicated techniques, be brought in mitosis.