(A) Western blot of ABCB4 protein expression in HEK293 cells transfected with eGFP or the natural coding sequences encoding murine ABCB4 (mABCB4wt, C219 antibody), or human ABCB4 (hABCB4wt, P3-II26 antibody). (B) PC-transporting activity in mock-, eGFP, or hABCB4 mRNA transfected HEK293 cells. PC species 34:1 (m/z 760.2) in the cell culture medium was determined as described in the Materials and methods. (C) Capillary electrophoresis immunoblotting for hABCB4 protein 24 h after transfection with 13 codon-optimised hABCB4 mRNAs variants (V1–V13). Densitometry analysis after normalisation to β-actin is shown below. (D) Enhanced PC-transporting activity by the codon-optimised hABCB4v6 mRNA (as denoted by the arrow in C) in transfected HEK293 cells. Data are mean ± SEM (from 3–4 independent experiments). **p<0.01, ****p<0.0001 (ANOVA followed by Tukey’s test). hABCB4, human ATP-binding cassette 4; MM, molecular mass marker; PC, phosphatidylcholine; WT, wild-type.