MUTAGEN SENSITIVITY IN HUMANS - A COMPARISON BETWEEN 2 NOMENCLATURE SYSTEMS FOR RECORDING CHROMATID BREAKS

Currently, there are two systems in use for recording chromatid breaks , either spontaneously occurring or induced by mutagens: The Internati onal System for Human Cytogenetic Nomenclature (ISCN) and the Chatham Barrs Inn Conference (CBIC) recommendation. The former system consider s that a chromatid break is recognized only when the chromatid fragmen t is displaced to the other side of its sister chromatid, while all ot hers, regardless of the distance between the two broken ends, are call ed chromatid gaps. The CBIC system recognizes a chromatid break when t he intervening achromatic segment is longer than or equal to the diame ter of the chromatid, whether the fragment is displaced or not. Minor lesions are called chromatid gaps. We conducted experiments using bleo mycin treatment of human cells (primary cultures or lymphoblastoid cel l lines) and read the chromatid lesions both ways. We conclude that th e CBIC system appears to have more direct biologic relevance than the ISCN system.