I looked up the 'classic' description of the amino acid sequence for lysozyme here: http://www.biology-pages.info/L/Lysozyme.html

Then I crossed referenced that with the lysozyme sequence on uniProt: https://www.uniprot.org/uniprot/B8YK79

My question is why does the sequence on uniprot start out with eighteen other amino acids before getting to the start of the sequence described in the textbook? 

uniprot starts out with "MRSLLILVLCFLPLAALG" I know the M comes from the starting codon in the gene for the protein, but what is the rest of this preamble from?