**Structural Distance Metric (SDM) as defined by Johnson and coworkers
(
J Mol Evol. 1990 Jan;30(1):43-59) is**

**SDM = -100 * LOG (W1*PFTE+W2*SRMS)**

**Where,**

**PFTE = Number of topologically equivalent residues**

**Sequence length of the smallest domain**

**SRMS = 1-(RMSD (in Å)/3.0)**

**W1 = (1-PFTE+1-SRMS)/2**

**W2 = (PFTE+SRMS)/2**

**Structural Distance metric following the work of Levitt and Gerstein
(
Proc Natl Acad Sci U S A. 1998 May 26;95(11):5913-5920) is defined as **

**SDM = -100* (1.0 - S)**

**Where S = SS-(GP/2) **

**SS = (1/N**_{res}**) * (S1/(1+(d**_{ij}**/3.0)**^{2}**),
dij = distance between topologically equivalent i**^{th}**
and j**^{th}** C-as
and N**_{res}** is the number of residue-residue
alignment positions.**

**GP = (N**_{gap}**/N**_{ali}**), N**_{gap}**
= number of insertions in deletions (this excludes the ones in the
N-terminal and C-terminal regions) and N**_{ali }**=
total alignment length (starting from the first residue to residue
alignment position to the last residue to residue alignment position).**