This tool allows to copy / paste directly an amino sequence as a one letter or 3 letters code. You may also use sequence copied directly from UniProt or Pdb like for example:

SEQRES   1 A 1020  MET LEU SER ALA HIS GLN TRP LEU ARG HIS CYS ILE ILE
SEQRES   2 A 1020  GLY VAL ALA SER VAL ALA LEU LEU GLN ALA CYS SER LYS 
SEQRES   3 A 1020  GLN THR GLY ASN GLU SER SER SER SER ALA LYS SER ALA
        10         20         30         40         50         60 
MRAMRLRDTT AVAHTLGRKV YSSLKRAAVN NLDLFAGGAA AGAVLLHLLD DVHAAGDLAE 
Molecular formula or peptide sequence:
Remove e- - 1e- - 2e- - 3e- - 4e- - 5e- n:
Add e- + 1e- + 2e- + 3e- + 4e- + 5e- n:
Add H+ + 1H+ + 2H+ + 3H+ + 4H+ + 5H+ n:
Remove H+ - 1H+ - 2H+ - 3H+ - 4H+ - 5H+ n:
Other + CH3+ + NH4+ + Na+ + K+
  Fragmentations (+):
Frag A Frag X
Frag B Frag Y
Frag C Frag Z
immonium ion
Resolution:     Reference values version: