LAMP1 is a protein primarily associated with the lysosomal membrane. Antibodies to LAMP1 are markers of lysosomes in mammalian cells, though some LAMP1 may also be seen on late endosomes and on the plasma membrane. In a typical cell LAMP1 is associated with spherical vesicles located next to the nucleus and the microtubule organizing center (1). The protein is also known as CD107a, LGP120 and LAMPA. LAMP1 is found on the cell surface of lymphocytes undergoing degranulation and this phenomena resulted in discovery of LAMP1 as a CD protein. The LAMP1 protein has a large N-terminal region which is inside the lysosome, hence topologically external to the cell, which is often refered to as the lumenal domain (2). The lumenal domain consists of two homologous globular seqments separated by a proline rich sequence. There is a single membrane spanning domain and a short 11 amino acid C-terminal cytoplasmic tail. This tail region contains a so-called YXXI motif which is responsible for the sorting of the intact molecule to the endosome and lysozome, where Y = tyrosine, I = isoleucine and X = almost any amino acid (3). This motif is found in several other lysosomal proteins, where it functions in the same way. There are 417 amino acids in the human LAMP1 molecule, giving a native molecular weight of 44.8kD. The N-terminal lumenal segment of LAMP1 is very heavily and variably glycosylated due to the presence of 18 N-linked glycosylation sites, so that on SDS-PAGE and on Western Blots the protein runs as a diffuse band at 90-120kD.