examined the behavior of chromosomes in wheat cells that had been subjected to X-rays. She noticed that the X-rays produced chromosomal breaks during G1 phase, and that after subsequent chromosome replication in S phase, the broken ends of the two sister chromatids could join together to make a fusion chromosome larger than the original. Even later, during mitotic metaphase and early in mitotic anaphase, the joined sister chromatids would form an unusual bridge structure in which chromatin was stretched between the two spindle poles and could then eventually break. She called this phenomenon the breakagefusion-bridge cycle. Each of photographs (a) and (b) that follow shows a cell in early mitotic anaphase that has two such chromatin bridges.