The Infection of Cells by the Human Immunodeficiency Virus (HIV)

The infection of cells by the human immunodeficiency virus (HIV) requires the virus to bind to the cell surface molecule CD3 and a second coreceptor molecule, CCR5 or CXCR4.In Scandinavian populations a mutation in CCR5 results in functional loss of CCR5 expression and exists at an allelic frequency of about 10 to 18 percent.Homozygous loss-of-function carriers are protected from infection by HIV strains that bind CCR5, and this mutation is also believed to have been protective against smallpox infection.This CCR5 mutation is evidence that mutations

A) can provide selective advantage to an organism.
B) can arise from errors in DNA replication.
C) are always caused by mutagens.
D) occur at the same rate for all bases.
E) are always deleterious.