Mex67p/Tap are evolutionally conserved mRNA export factors. To identify mutations in genes that are functionally linked to mex67 with respect to mRNA export, we used a synthetic lethal genetic screen in Schizosaccharomyces pombe. Three synthetic lethal mutants were isolated and mutations in these mutants defined separate complementation groups. These mutants exhibited the accumulation of poly A$^$+/ RNA in the nucleus, with a decrease in the cytoplasm under synthetically lethal conditions, suggesting that the mutations cause an mRNA nuclear export defect. In addition, the S. pombe genes that were found to be involved in mRNA export did not suppress the synthetic lethality of these mutants. These results indicate that the isolated mutants contain mutations in new genes, which are involved in mRNA export from the nucleus.