Utilize the input text as a context reference, choose the right relationship between Boris Johnson and Mayor of London from the options. Options: product/material produced, manufacturer, distributed by, industry, position held, original broadcaster, owned by, founded by, distribution format, headquarters location, stock exchange, currency, parent organization, chief executive officer, director/manager, owner of, operator, member of, employer, chairperson, platform, subsidiary, legal form, publisher, developer, brand, business division, location of formation, creator.

Welcoming the investment, the Mayor of London Boris Johnson said: "This is tremendous vote of confidence in London's thriving life sciences sector as well as the skills and talent of our amazing healthcare professionals.

Answer: position_held