The objectives of Mendelian Cytogenetics Network (MCN) are to collect data on Disease Associated Balanced Chromosomal Rearrangements (DBCRs), both retro- and prospectively; to facilitate the dissemination of these data by an online database (mcndb) containing the information on breakpoints and clinical features; identify those DBCRs with the highest disease potential in order to facilitate cost-efficient clinical reexamination and generation of material (cell lines, chromosome suspensions for FISH mapping, DNA); distribute standardized FISH probes for the molecular mapping of DCBRs; initiate collaborative studies of specific disorders; provide facilities and contacts for cell line production, FISH-mapping and molecular cloning of DBCRs for those laboratories who may wish it.