Researchers have long known that some repetitive DNA sequences can make human chromosomes "fragile," i.e. appearing constricted or even broken during cell divisions. Scientists at Tufts University have found that one such DNA repeat not only stalls the cell's replication process but also thwarts the cell's capacity to repair and restart it. The researchers focused on this CGG repeat because it is associated with hereditary neurological disorders such as fragile X syndrome and FRAXE mental impairment.

In his book, Wonderful World, Stephen Jay Gould writes about an experiment of 'replaying life's tape', wherein one could go back in time, let the tape of life play again and see if 'the repetition looks at all like the original'. Evolutionary biology tells us that it wouldn't look the same – the outcome of evolution is contingent on everything that came before. Now, scientists at the Instituto Gulbenkian de Ciência (IGC) in Portugal, New York University and the University of California Irvine, provide the first quantitative genetic evidence of why this is so.