Full-bandwith spectrum of APM 08279 in the region around CO(4−3). The spectrum was spatially integrated over the source, after continuum subtraction, over a polygon around the source, determined by the 2σ contour level in a line map integrated over the velocity range −1340 to +670 km s^-1. The observed frequency is reported on the upper x-axis. The continuum has been subtracted in the uv plane as detailed in Sect. 3.2. Labels marking spectral features are just to guide the eye. Actual line identifications are discussed in Sect. 4.