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Small stretches of seemingly useless DNA harbor a big secret, say researchers at the Stanford University School of Medicine. There's one problem: We don't know what it is. Although individual laboratory animals appear to live happily when these genetic ciphers are deleted, these snippets have been highly conserved throughout evolution.
It's not that these regions are somehow protected against change: they are mutated in about one in 200 healthy humans. Rather, these changes seem to be swept away over time by the tides of evolution in a process called "purifying selection." Bejerano and McLean believe that something similar may be happening in the laboratory mice on a scale too subtle to be seen under carefully controlled experimental conditions.
"Interestingly," said Bejerano, "the longer the sequence has been in us, the less likely it is to be lost. It's almost like the bricks in the foundation of a building, which hold up the rest of the structure."