Describe the process of generating stable cell lines for protein expression.

Stable cell line generation involves: transfection with expression construct containing selectable marker, selection using appropriate drug (G418 for neomycin, puromycin, hygromycin), single-cell cloning (limiting dilution or FACS sorting), expansion and screening of clones for expression level, and banking of high-expressing clones. Integration site affects expression stability (random integration vs targeted insertion using CRISPR). Amplifiable markers like DHFR (with methotrexate selection) or GS (with MSX) enable copy number amplification for higher expression. Pool vs clonal approach depends on timeline and expression requirements. Characterization includes productivity, growth rate, and stability testing over passages.