A putative MRP4 polypeptide sequence (GenBank ID: GAA49862.1) of C. sinensis was retrieved from the National Center for Biotechnology Information (NCBI) database. The coding DNA sequence (CDS) was obtained from the C. sinensis DNA scaffold (GenBank ID: DF142991.1) to which it belonged. DNA-walking was performed twice for CsMRP4-I and CsMRP4-II due to the long size (approximately 4 kb) of the products. Two sets of PCR primers were designed according to CDS and synthesized (Bioneer, Daejeon, Korea) (Additional file 1: Table S1). Total cDNA of C. sinensis was prepared as described previously , and 50 ng per reaction was used as the template for DNA-walking. PCR amplification was performed under the following conditions: pre-denaturation (94 °C for 5 min), amplification phase with 35 cycles (94 °C for 30 s, 55 °C for 30 s, 72 °C for 2 min 15 s), and final extension (72 °C for 10 min). The PCR products were then purified using the QIAquick PCR purification kit (Qiagen, Hilden, Germany) and sequenced (Macrogen Inc., Seoul, Korea). The CsMRP4-I and CsMRP4-II sequences were used for assembling the putative CDS and translated into amino acid (aa) sequences. However, CsMRP4 was assumed to be incomplete at the 5′-end upon comparison with MRP4 of other species. Therefore, 5′-rapid amplification of the cDNA ends (5′-RACE) was carried out to obtain the entire CDS. Total cDNA of C. sinensis was synthesized using the SMARTer™ RACE cDNA amplification kit (Clontech, Mountain View, CA, USA) according to the manufacturer’s instructions. The missing 5′-end of CsMRP4 was amplified by RACE-PCR run using the 5′-RACE universal primer mix (UPM) and gene specific reverse primer (GSP). The PCR product was then confirmed using nested PCR, purified, and subjected to TOPO TA cloning (Invitrogen, Carlsbad, CA, USA). Through blue-white screening, the positive white colony was selected and reconfirmed by rapid colony PCR. Its plasmid DNA was extracted using the Plasmid Miniprep kit (Qiagen, Seoul, Korea) and sequenced (Macrogen Inc., Seoul, Korea). The primers used for RACE-PCR, DNA-walking, and multiple sequencing are listed in Additional file 1: Table S1.