The number specifies the sequence-identity threshold used when clustering related sequences before deriving a BLOSUM substitution matrix. At 62%, sufficiently similar sequences are grouped and their within-cluster comparisons are excluded. Lowering the threshold merges more sequences, emphasizing comparisons across greater divergence.
Consequently BLOSUM80 is suited to closer relatives, BLOSUM45 to more divergent sequences, and BLOSUM62 is intermediate. The numbers do not specify a required identity of the query and target, nor a number of evolutionary time steps.[ref]
Amino acid substitution matrices from protein blocks by Steven Henikoff and Jorja G. Henikoff. pp. 10916–10917, clustering levels and performance across divergence
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