Several linkage studies suggest the involvement of specific chromosomal regions in the vulnerability to develop heroin addiction (Gelernter et al., 2006, Glatt et al., 2006, Glatt et al., 2008). The initial linkage study by the Tsuang group using 192 Chinese families and 386 short tandem repeat (STR) markers found a region on chromosome 4 at D4S1644 with evidence for linkage with heroin dependence (point-wise P = 0.014) (Glatt et al., 2006). In their follow-up study, which included the original 192 families with a total of 397 Chinese families and 385 STR markers, the linkage signal on chromosome 4 at D4S1644 with greater significance (point-wise P = 0.004) (Glatt et al., 2008). Variant rs10518620, ranked seventh in Table 2 in our study and nominally associated with heroin addiction in the Caucasian group (point-wise P = 0.0003), is located between D4S1644 and the nearby marker D4S2394, which had nominal significance (point-wise P = 0.013) in the later Tsuang study. In the linkage study of Gelernter, which used 409 STR markers in 393 families, a linkage peak at D17S785 was found (LOD =
