PURPOSE. POAG is a complex disease; therefore, families inwhich a glaucoma gene has been mapped may carry additionalPOAG genes. The goal of this study was to determine whethermutations in the myocilin (MYOC) gene on chromosome 1 arepresent in two POAG families, which have previously beenmapped to the GLC1C locus on chromosome 3.METHODS. The three exons of MYOC were screened by denaturing(d)HPLC. Samples with heteroduplex peaks were sequenced.Clinical findings were compared with genotype statusin all available family members over the age of 20 years.RESULTS. A T377M coding sequence change in MYOC wasidentified in family members of the Greek GLC1C family butnot in the Oregon GLC1C family. Individuals carrying both theMYOC T377M variant and the GLC1C haplotype were moreseverely affected at an earlier age than individuals with just oneof the POAG genes, suggesting that these two genes interact orthat both contribute to the POAG phenotype in a cumulativeway.