The gene for glucocerebrosidase ( GBA), the enzyme deficient in Gaucher disease, is located in a gene-rich region on 1q21. Metaxin 1( MTX1) is a convergently transcribed gene contiguous to the 3' end of the GBA pseudogene. A single nucleotide alteration in MTX1, 628T-->C, resulting in the amino acid change F202L, was identified in patients with Gaucher disease in association with the common N370S mutation in GBA. The polymorphism was also present on 4.6% of 152 control alleles, but could have functional consequences that have a modifying role in Gaucher disease.