1. Semiquantitative RT-PCR analysis: We extracted total RNA from cells with TRIZOL reagent (Invitrogen), according to the manufacturer’s instructions. RT-PCR experiments were carried out with cDNAs generated from 2 µg of total RNA using a GeneAmp RNA PCR kit (Applied Biosystems, Foster City, CA). The RT-PCR exponential phase was determined on 22-30 cycles to allow semiquantitative comparisons of cDNAs developed from identical reactions with TaKaRa ExTaq polymerase (TaKaRa Bioscience, Kyoto, Japan). The primers and PCR conditions are shown in supplementary Tables 1 and 2, respectively.