  Phosphorus is a crucial element in biochemistry, especially the P-O bond,
which is key for the formation of the backbone of the deoxyribonucleic acid. So
far, PO has only been detected towards the envelope of evolved stars, and never
towards star-forming regions. We report the first detections of PO towards two
massive star-forming regions, W51 e1/e2 and W3(OH), using data from the IRAM
30m telescope. PN has also been detected towards the two regions. The abundance
ratio PO/PN is 1.8 and 3 for W51 and W3(OH), respectively. Our chemical model
indicates that the two molecules are chemically related and are formed via
gas-phase ion-molecule and neutral-neutral reactions during the cold collapse.
The molecules freeze out onto grains at the end of the collapse and desorb
during the warm-up phase once the temperature reaches 35 K. Similar abundances
of the two species are expected during a period of 5x10^{4} yr at the early
stages of the warm-up phase, when the temperature is in the range 35-90 K. The
observed molecular abundances of 10^{-10} are predicted by the model if a
relatively high initial abundance of 5x10^{-9} of depleted phosphorus is
assumed.
