Review of Packaging Cell Lines 
Introduction 
Several investigators (Cone and Mulligan. 1984; Miller et al.. 1985; 
Miller and Buttimore 1986) have constructed packaging cell lines with a wide 
host range specificity. The packaging cell has an integrated provirus which 
itself cannot be packaged because of a deletion in its packaging sequence in 
cis . but which encodes in trans all of the viral proteins necessary for 
packaging a vector introduced into the cell by infection or transfection. The 
construction of the packaging line is delineated in the following section. The 
production of high titer virus, the detection of helper virus, and the 
description of an improved packaging line which theoretically is helper-free 
are al so described. 
Construction of PA-12 
Synopsis 
To construct PA-12 the following steps were undertaken. A wild type 
integrated Moloney Murine Leukemia Virus (Mo-MuLV) provirus. pMLV-1. which had 
been cloned and sequenced, was used as the backbone of the construct. As 
pMLV-1 was not biologically active, a second clone of a Mo-MuLV. pMLV-48. which 
is biologically active, was used to determine the region of the viral genome 
responsible for the inactivity of pMLV-1. This region in pMLV-1 was identified 
and replaced by the homologous region in pMLV-48 to render the virus 
infectious. The new construct, yielding high titer infectious virus, was 
called pMLV-K. Subsequently, the envelope gene region of pMLV-K was replaced 
by the homologous region of an amphotropic, or wide host range, murine virus 
called 4070A, creating pAM. a virus with wide species specificity. Finally, 
the region in pAM responsible for conferring in cis the ability of the virus to 
be packaged, was deleted, and the contiguous ends religated, creating pPAM. 
f 166 l Recombinant DNA Research, Volume 12 
