Recombinant CD33 -CHO K1 cell line stably expressing the full length human C33/Siglec-3 receptor (GenBank Accession: BC028152.1). Surface expression of CD33 was confirmed by flow cytometry. Each stable clonal cell line was selected for different levels of CD33 expression (High, Medium, Low) to mimic different stages of cancer target cells with various CD33 expression levels.