A common example used in Functional Medicine circles revolves around the MTHFR alleles. If one is homo or heterozygous, they run the risk of insufficient methylation. Given folates role in methylation, a 'functional' marker of poor methylation (activated genetic vulnerability) may be elevated levels of forminoglutamic acid.
From Diagnostic Function Tests in Chemical Pathology
Dietary L-histidine is converted to formiminoglutamic acid (FIGLU), which is then removed from the tissues by reacting with tetrahydrofolic acid. In the presence of folate deficiency, FIGLU cannot be metabolised; a large quantity, therefore, is excreted in the urine unaltered, measurement of which indicates the extent of folate depletion.