Two variants in SLCO1B1, rs4149081 and rs11045879, have been identified as being associated with MTX-related toxicity and MTX clearance in a genome-wide analysis study.13 Ramsey et al. confirmed that the SLCO1B1 variants, rs4149056 and rs2306283, were predictors of MTX clearance in patients with ALL.14,15 In the present study, associations between SLCO1B1 rs4149056 and MTX serum level and elimination delay were established, a result similar to that of den Hoed et al.16 Additionally, an increased risk of neutropenia was associated with rs4149056 variant (TC or CC genotype). The gene discussed is SLCO1B1; the disease is neutropenia.