Methionine synthase is a key enzyme poised at the intersection of folate and sulfur metabolism and functions to reclaim homocysteine to the methionine cycle. The 5' leader sequence in human MS is 394 nucleotides long and harbors two open reading frames (uORFs). In this study, regulation of the main open reading frame by the uORFs has been elucidated. Both uORFs downregulate translation as demonstrated by mutation of the upstream AUG codons (uAUG) either singly or simultaneously. The uAUGs are capable of recruiting the 40S ribosomal complex as revealed by their ability to drive reporter expression in constructs in which the luciferase is fused to the uORFs. uORF2, which is predicted to encode a 30 amino acid long polypeptide, has a clustering of rare codons encoding arginine and proline. Mutation of a tandemly repeated rare codon for arginine at positions 3 and 4 in uORF2 to either common codons for the same amino acid or common codons for alanine results in complete alleviation of translation inhibition. This suggests a mechanism for ribosome stalling and demonstrates that the cis-effects on translation by uORF2 is dependent on the nucleotide sequence but is apparently independent of the sequence of the encoded peptide. This study reveals complex regulation of the essential housekeeping gene, methionine synthase, by the uORFs in its leader sequence.