A schematic representation of OPRM1 Gene Structure, Linkage. Disequilibrium and Genotyped SNPs. The gene structure of OPRM1 is shown with exons numbered from 1 to 4 and relative exon size denoted by the width of the vertical bars. Thirteen SNPs analyzed in this study are shown in relation to their location across OPRM1. Linkage disequilibrium (LD; shown below the gene structure) data, as measured by the correlation coefficient r2 statistic, was generated using Haploview . LD causes tightly linked genetic variants to be highly correlated. Shading represents correlation magnitudes between low r2 (white) and high r2 (red).