The mortality risk of genotypes in longitudinal data was estimated with STATA 11.1 (Stata Corporation, College Station, TX, USA) using a sex-adjusted, left-truncated Cox proportional hazards model to adjust for late entry into the dataset according to age. Because intact twin pairs were included in the study, statistical analyses were performed using the robust estimator of variance, assuming independence between the twin pairs. The proportional hazard assumption was evaluated using Schoenfeld residuals and performing an Aalen linear hazard model, and both suggested a change in effect with age for rs2853668, rs33954691, rs3772190, and rs3891054. Hence, an extended Cox model was performed splitting the effect up into age spans which the Aalen model supported.

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