A gene, whose mutation plays a significant role in a disease. In some diseases such as sickle cell anemia, the substitution of a single base is sufficient to cause a dysfunctional protein, thus leading to the onset of the disease.
In other diseases such as cancer, Alzheimer's or rheumatism, the mutation of a disease gene does not lead to the onset of the disease in every case, but simply increases the susceptibility to a certain disease.