Illustration of the spike protein (red) from the B.1.617 variant of the SARS-CoV-2 coronavirus, the cause of the respiratory illness Covid-19. The mutation sites of the variant are labelled. The spike protein binds to angiotensin converting enzyme 2 (ACE2, blue) in the host cell membrane, mediating viral entry to the host cell. This variant was first identified in India. A sublineage of this variant, known as the Delta (B.1.617.2) variant is a Variant of Concern, as it spreads much more rapidly than earlier variants.