What is the relationship between Boris Johnson and Mayor of London in the context of the input sentence. Choose an answer from: 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