In the past, basic research into HBV and the development of antiviral therapeutics have both been hindered by the lack of suitable in vitro infection systems and animal models. Now, the work of Yan et al. means that it will be possible to use NTCP-complemented HepG2 cells for challenges as diverse as fundamental studies of basic viral entry/replication mechanisms and large-scale drug screening. It is also possible that HBV and HDV infection might interfere with some of the important physiological functions carried out by NTCP, so the latest work could also be of interest to medical scientists working on other diseases related to these infections. Research article Figure 1 . Developing photoreactive peptide ligands and an antibody for identifying pre-S1 binding partner(s) by zero distance cross-linking. (A) Schematic diagram of HBV envelope proteins and N-terminal peptides of pre-S1 domain. Pre-S1 (2-47): 2-47 th residues of the pre-S1 domain of the L Figure 1 . Continued on next page nonnatural amino acids (L-photo-leucine, L-2-amino-4,4-azi-pentanoic acid) ( Figure 1A) . L-photoleucine contains a photoactivatable diazirine ring. Irradiation of ultraviolet (UV) light at 365 nm induces a loss of nitrogen of the diazirine ring and yields a reactive carbene group with short half-life for covalent cross-linking at nearly zero distance (Suchanek et al., 2005) . Primary hepatocytes isolated from treeshrews (Tupaia belangeri), the only species susceptible to human HBV infection other than humans and chimpanzees (Su et al., 1987; Walter et al., 1996; Glebe et al., 2003) , were used as target cells.