Gender (HR, 0.893; P = .107), rs11886868 in BCL11A (HR, 078; P = .061), rs766432 (HR, 0.81; P = .109), rs4671393 (HR, 0.98; P = .893), rs4895441 (HR, 0.91; P = .395), and -29(A>G) (HR, 0.89; P = .594) were removed from the model. The discriminative ability of the model was high (Harrell concordance index = 0.714, R² = 0.344).

Multivariate analysis was performed using a backward stepwise Cox proportional hazards model in 1142 β-thalassemia homozygotes or compound heterozygotes in cohort A, and HR with 95% CI was obtained to evaluate the associations between putative ameliorating alleles and the patient's age at first transfusion. All covariates were classified as follows: α-globin gene mutations as 0, 1, 2, or 3 based on the number of mutated copies of the HBA; for other loci, each variable was defined with values 0, 1, or 2 according to the number of copies of putative modifying allele: HBB mutations CD41-42(-CTTT), CD17(A->T), IVS-II-654(C>T), CD71-72(+A), IVS-1-1(G>T), CD27-28, CD43, HBB:c.-78A>G, -29(A>G), and IVSII-5, HBA. The model's performance was measured by Harrell concordance index using the rms package in R version 3.0.1 (http://www.r-project.org/).

95% CI, 95% confidence interval.