""Scientists suspect that one mutation, called N501Y, is very important in making B.1.1.7 coronaviruses more contagious. The mutation’s name refers to the nature of its change: the 501st amino acid in the spike protein switched from N (asparagine) to Y (tyrosine).""

https://www.nytimes.com/interactive/2021/health/coronavirus-mutations-B117-variant.html