These viruses allowed us to efficiently elucidate the role of some host factors in PRRSV infection. Several infectious cDNA clones have been generated from field PRRSV strains [31] [32] [33] [34] [35] [36] [37] , which provide efficient means for molecular manipulation of PRRSV genome and evaluation of molecular evolution of this RNA virus with a high mutation rate. The vector cassette used in this study was generated from an infectious clone of North American PRRSV strain P129 with two unique restriction sites and a copy of the transcription regulatory sequence of ORF6 (TRS6) inserted between ORFs 1b and 2a [26, 27, 29, 30] . In addition to GFP, which was introduced in PRRSV infectious clones in several previous studies [26, 27, 29, 31] , two other indicator proteins of DsRed and Rluc were introduced in the PRRSV cDNA clone in this study. The construction of DsRed and particularly Rluc into the PRRSV cDNA clone was intended to produce laboratory viruses that mimic their parental PRRSV with similar replication kinetics but are easily detected and quantified during the early phase of virus infection/replication. As shown in both MARC-145 cells and PAMs, all GFP, DsRed and Rluc expressing progeny viruses had similar replication kinetics as their parental PRRSV. For detection, the red fluorescence of DsRed labeling not only provided a counterstaining choice but also was more sensitive for microscopic observation in real-time rather than traditional immunostaining procedures in fixed cells (Rural Technologies, Brookings, SD, USA). In contrast to GFP and DsRed viruses, which were generally detectable after 12 h in infected cells using microscopy, the Rluc PRRSV in conjunction with an EnduRen™ in vivo substrate (Promega, Madison, WI) facilitated real-time detection of the virus replication as early as 5 h post infection in cells. This Rluc expressing PRRSV thus provides an efficient means for genome-wide examination of host factors involved in the early stages of virus infection [38, 39] . However, our attempts to clone the firefly luciferase gene (~1.6 kb) into the same PRRSV cDNA vector were unsuccessful, suggesting that the PRRSV clone vector has a limited capacity for incorporation of exogenous genes at about 2 kb. 