After more than a decade and a half in Cheshire, Alexion Pharmaceuticals will begin relocating its employees to its new headquarters in New Haven.

Kim Diamond, Alexion’s executive director of corporate communications, said the relocation will be done in phases. “By the end of March, we will have 1,000 people working in New Haven,” Dia­mond said.

Alexion is a bio-pharmaceutical company focused on developing drugs to treat people who have rare diseases. The company was founded in 1992 and was based in New Haven’s Science Park complex until 1997, when it relocated to Knotter Drive in Cheshire.