In the UK, the Human Fertilisation and Embryology Authority has licensed a number of tightly-controlled research projects into mitochondrial diseases.

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The best-known example is Salesforce.com, which is licensed according to the number of users on a subscription basis.

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The number of licensed PAs, meanwhile, has doubled in the past decade, to 86, 500, and is likely to grow another 30% by 2020, according to the American Academy of Physician Assistants.

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