Current estimation is that there are 1,000 – 100,000 genes per organism, and 10^11 to 10^13 different proteins. These proteins are composed of one or more fold domains.
According to the difference in their secondary structure connectivities and spatial arrangements, current estimation of the number of different protein folds are less than ten thousand. Protein Fold space incorporates all possible 2nd structure organizing ways that can be assumed by a polypeptide chain. Currently available protein structure has shown non-uniform dist in fold groups. It is conceivable to expect only a portion of protein fold space is occupied by natural existing proteins.