The receptor defect is an autosomal recessive mutation or polymorphism. This can be caused by a single point mutation or multiple mutations. Rett syndrome is due to a genetic mutation of the MECP2 gene. Genetic testing revealed the MTHFR C677T mutation. They study processes such as mutation and gene transfer that drive evolution. The bobtail is due to the mutation of a dominant gene. Several subtypes exist, depending on the exact mutation in each haemoglobin gene. The cottony coat may be the result of a single gene mutation. Some viruses escape the anti-viral activities of interferons by gene (and thus protein) mutation. It is an example of a cat body type genetic mutation. Some cases may be due to a new genetic mutation or certain medications. Opinions vary as to whether this trait might be the result of genetic mutation. Polyploidy is an example of chromosomal mutation. Individuals heterozygous for a point mutation in FOXP2 manifest a speech disorder. The study looked at heterozygous "MYBPC3" mutation in human embryos.