MEROPS is an online database for peptidases (also known as proteases, proteinases and proteolytic enzymes) and their inhibitors.
Typically, FastA or BlastP is used to establish sequence relationships, with an expect value of 0.001 or lower taken to be statistically significant.
Each family is identified by a letter representing the catalytic type of the peptidases it contains followed by an arbitrary unique number.
Typically, DALI is used to establish clan membership, with a z score of 6.00 standard deviation units or above considered to be statistically significant.
For peptidases, other evidence to indicate that families are related when a tertiary structure is absent includes the same order of catalytic residues in the sequences.