In fact, even heterozygous mutations which lead to fewer co-receptors on the cells, can make it harder for the virus to spread, and results in a slower disease progression.
It's autosomal dominant, which means that even if there's a normal copy of the gene, a single mutated copy of the gene – in other words a heterozygous mutation - is sufficient to cause the disease.
Using these tests, parents who are both heterozygous for a mutation can now be advised whether an unborn child is going to suffer from a genetic disorder such as muscular dystrophy or cystic fibrosis and can consider termination.