If minor chromosomal alterations (microdeletions or microduplications) are suspected, it is possible to clarify these specifically using fluorescence in situ hybridization (FISH). In certain cases (e.g. multiple malformations, developmental delay) and suspected small chromosomal aberrations, all chromosomes can be examined using high-resolution molecular karyotyping (array diagnostics).

Fluorescence in situ hybridization (FISH) makes it possible to stain defined chromosomal regions. The examination can be carried out on metaphase chromosomes or interphase cell nuclei. The DNA of the FISH probes is labeled with fluorescent dyes. During hybridization, the probe binds to the homologous chromosome region. To clarify a structural chromosome disorder, probes that stain the entire chromosome or locus-specific probes can be used, for example. The clarification of microdeletions and duplications is of particular clinical importance here.