Common Disease - Common Variant

Common disease - common variant (CD-CV) hypothesis says that the common disease-causing allele (or variant) will be found in all human populations commonly having that disease.

Common variants (not necessarily disease-causing) are known to exist in coding and regulatory sequence of genes. CD-CV says, some of these variants lead to cause that disease.

Common example is SNP (Single Nucleotide Polymorphism) - single nucleotide base change in DNA. SNP variants tend to be common in different human population. This polymorphism have been valuable as "markers," in search for common variants causing a common disease.

In complex disease, the effect (additive or multiplicative) of a variant at a gene to cause the disease will be very small and it will be evolutionarily neutral, as so many genes influence a complex disease.