HEK293 were originally established by transfecting human embryonic
kidney cells with sheared adenoviral DNA. The cells express several
adenoviral proteins, including the oncogenes responsible to
inactivation of p53 and pRB. They do not make adenenovirus, but
support replication of adenoviral vectors.
HEK293T have been additionally transfected with an expression
construct for large-T antigene of SV40 virus. Several cell lines were
established this way. The chances are, you have the one that was
established with the temperature-sensitive large T. Large T affects a
multitude of processes in a cell, and some of its functions duplicate
those of adenoviral oncogenes. However, it is also a transcription
factor and a replication factor for SV40 in primate cells. The
original intent was to have a system where transcription and/or
replication could be regulated in mammalian cells in a LgT-dependent
manner. Superb transfection efficiency is what made these cells
popular.
T-REx™-293 are a commercial product of Invitrogen. They were derived
from HEK293 by stable expression of and expression cassette for the
modified tet-repressor protein (from pcDNA6/TR plasmid).
Eugene.