DNAs encoding full-length human α1 and α2 were amplified with primers designed to encode a 5′ Kpn1 site, and a 3′ FLAG-tag followed by an Xho1 site. The resulting PCR products were cloned into the pcDNA5/FRT plasmid (Invitrogen). Stable cell lines were generated and cultured as described previously [19 (link)].