  We present the first unbiased molecular line survey towards an extragalactic
source, namely the nuclear region of the starburst galaxy NGC 253. The scan
covers the frequency band from 129.1 to 175.2 GHz, i.e. most of the 2mm
atmospheric window. We identify 111 spectral features as transitions from 25
different molecular species. Eight of which (three tentatively) are detected
for the first time in the extragalactic interstellar medium. Among these newly
detected species, we detected the rare isotopomers 34SO and HC18O+. Tentative
detections of two deuterated species, DNC and N2D+, are reported for the first
time from a target beyond the Magellanic Clouds. Additionally, three hydrogen
recombination lines are identified, while no organic molecules larger than
methanol are detected. Column densities and rotation temperatures are
calculated for all the species, including an upper limit to the ethanol
abundance. A comparison of the chemical composition of the nuclear environment
of NGC 253 with those of selected nearby galaxies demonstrates the chemical
resemblance of IC 342 and NGC 4945 to that of NGC 253. On the other hand, the
chemistries characterizing NGC 253 and M 82 are clearly different. We also
present a comparison of the chemical composition of NGC 253 with those observed
in Galactic prototypical sources. The chemistry of NGC 253 shows a striking
similarity with the chemistry observed toward the Galactic center molecular
clouds, which are thought to be dominated by low-velocity shocks. This
resemblance strongly suggests that the heating in the nuclear environment of
NGC 253 is dominated by the same mechanism as that in the central region of the
Milky Way.
