oteins are basic blocks of any tissue. Protein structures can be classified into 4 levels of structural organization. Primary level is the amino acid structure. The remaining secondary, tertiary and quaternary structure refers to the peptide arrangement in the polypeptide backbone, three dimensional structure of globular protein and structural aggregates of globular proteins respectively. Formaldehyde reacts with primary amines to form Schiff bases, with amides to form hydroxymethyl compounds. Hydroxymethyl groups condense with another amide moiety to form methyl daimides. Alcoholic hydroxyl forms acetals while sulfhydro groups form sulfhydral acetal analogues with formaldehyde.[6] Based on the affinity to combine with formaldehyde, proteins could be classified as strong affinity, moderate and less affinity ones. Presences of tyrosine rings, in proteins, have been identified as an important factor for the affinity of the protein to formaldehyde. In its absence, the presence of arginine residue, phenylalanine or tryptophan as a conserved substitution (tyrosine to a phenylalanine or tryptophan) would help to create the formalin affinity.[4]