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Did you know that the cell copies 50 nucleotides (letters of DNA code) per second when it is dividing? And it only makes one mistake per 100 million nucleotides! That’s like copying the full 32 volumes of Encyclopedia Britannica twelve times and only making one typo!

Most times even these mistakes are caught and fixed. But sometimes a mutation (mistake in the code) gets passed on. In eggs and sperm that means an unborn baby will get one bad copy of that gene.

In most cases, even this is okay. The baby is a carrier of a bad copy of the gene, but often the good copy from the other parent will work well enough. In rare cases, though, a baby may receive a bad copy from both parents. This means they will have a genetic disease. 
There are several diseases that are caused by a single nucleotide mutation. Scientists have always wanted to use genetic editing to correct the bad part of the gene. We found a way to do it in real, live mice!

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