It is a single nucleotide polymorphism (SNP) in the COMT gene that codes catechol-O-Methyltransferase.
The single nucleotide substitution between G--> A results in an amino acid change from valine to methionine at codon 158.
The A or Met allele is associated with lower enzymatic activity (due to thermoinstability), and with exploratory behaviour.
The polymorphism has been much studied in schizophrenia research but as of November 2011 meta-analysis in the SzGene database shows no or very little effect.
[1] Several personality genetics studies have examined the association of the polymorphism with personality traits including extroversion, risk aversion, and novelty seeking.