Healthy women who have a family history of breast cancer could be offered drugs on the NHS to cut their chances of contracting the disease.
Dr Hilary told ITV Daybreak that the offer of medication is a 'historic breakthrough' against cancer prevention.
Currently a draft consultation, he added that the challenge lies with the ability to roll these drugs out across the country.
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Sir John rejected the PM's Brexit vision as "unreal and over-optimistic" as he accused negotiators of "souring" the atmosphere for talks.
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White House budget officials outlined the information during a telephone call with reporters given on condition of anonymity.