All seven DSM-IV ND symptoms were included. Briefly, an individual who endorsed one of the dependence criteria was coded as “1” while those that did not endorse the symptom were coded as “0”. A sum of all seven symptoms was constructed, such that each individual scored between zero and seven. Non-smokers were coded as having zero ND symptoms. ND symptom counts were standardized in the CADD community sample with residuals from sex, age, and age2 based on a linear regression run in SAS (SAS Institute Inc., Cary, NC). The β-coefficients established from the CADD community sample were then applied to the CADD clinical and GADD samples. The correlation between sum number of ND symptom counts and the final standardized score was 0.98 in CADD and 0.99 in GADD (both p-values < 0.001). This score was used as the primary dependent variable in a family-based association test carried out in the FBAT software package using an additive model (Laird et al. 2000). To correct for the testing of multiple SNPs, the min –p option in FBAT was used (Rakovski et al.
