Amino acid sequences can be described as foldable or un-foldable depending on the nature of the tertiary structures they produce. Thus, understanding what makes a sequence foldable or un-foldable is crucial not only for classifying the huge number of sequences being produced by the various genome projects but also for understanding how amino acid sequence determines tertiary structure, that is, solving the protein folding problem. Through systematic circular permutation analysis of a small globular protein, dihydrofolate reductase, an idea of folding element has been introduced and led us to the conclusion that a complete set of folding elements, which leads to a collapse of the molecule in the early stages of folding, is required for a protein to be foldable.