It has been shown by a variety of methods (circular dichroism, viscosimetry, intrinsic fluorescence, and fluorescence of labels) that, as in the case of small globular proteins the folding-unfolding transition in the Caf1(13-149) subunit under the action of two denaturants (urea and 1,3-dimethylurea) occurs between two major states (unfolded and compact). However, the free energy of the compact structure is only 8/9-9/2 kJ/mol (similar values for single-domain small proteins are in the range of 21-63 kJ/mol).