Common variants (not necessarily disease-causing) are known to exist in coding and regulatory sequences of genes.
The hypothesis has held in the case of putative causal variants in apolipoprotein E, including APOE ε4, associated with Alzheimer's disease.
[1] IL23R has been found to be associated with Crohn's disease; the at-risk allele has a frequency of 93% in the general population [citation needed].
One common form of variation across human genomes is called a single nucleotide polymorphism (SNP).
Research has linked common SNPs to diseases such as type 2 diabetes, Alzheimer's, schizophrenia and hypertension.