The present invention provides isolated an polynucleotide comprising a polymorphic nucleotide sequence from a diacylglycerol acyltransferase (DGAT) gene, wherein at least one a polymorphism is associated with a condition relating to DGAT activity. The invention further provides diagnostic assays for detecting the presence in a nucleic acid sample of a polymorphism in a DGAT gene that is associated with a condition relating to DGAT activity, and for detecting a propensity to develop a condition associated with DGAT activity. The invention further provides treatment methods.