Mutagen sensitivity in humans: A comparison between two 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 International System for Human Cytogenetic Nomenclature (ISCN) and the Chatham Barrs Inn Conference (CBIC) recommendation. The former system considers that a chromatid break is recognized only when the chromatid fragment is displaced to the other side of its sister chromatid, while all others, regardless of the distance between the two broken ends, are called chromatid gaps. The CBIC system recognizes a chromatid break when the intervening achromatic segment is longer than or equal to the diameter of the chromatid, whether the fragment is displaced or not. Minor lesions are called chromatid gaps. We conducted experiments using bleomycin treatment of human cells (primary cultures or lymphoblastoid cell lines) and read the chromatid lesions both ways. We conclude that the CBIC system appears to have more direct biologic relevance than the ISCN system.