Molecular cytogenetics involves the combination of molecular biology and cytogenetics. In general this involves the use of a series of techniques referred to as fluorescence in situ hybridization, or FISH, in which   DNA probes are labeled with different colored fluorescent tags to visualize one or more specific regions of the genome. FISH can either be performed as a direct approach to metaphase chromosomes or interphase nuclei. Alternatively, an indirect approach can be taken in which the entire genome can be assessed for copy number changes using virtual karyotyping. Virtual karyotypes are generated from arrays made of thousands to millions of probes, and computational tools are used to recreate the genome in silico.