The ABCB1 1236 T allele, ABCB1 3435 T allele, ABCB1 2677 T allele, ABCB1 3435, 1236, 2677 TTT haplotype and the ABCG2 421 A allele were correlated with a 3-fold (P = 0.03), 10-fold (P = 0.01), 3-fold (P = 0.04), 10-fold (P = 0.03) and 3-fold (P = 0.03) decrease in the risk of neutropenia respectively. This evidence concerns the gene ABCB1 and neutropenia.