All old Galactic Globular Clusters studied in detail to date host at least two generations of stars, where the second is formed from gas polluted by processed material produced by massive stars of the first. This process can happen if the initial mass of the cluster exceeds a threshold above which ejecta are retained and a second generation is formed. A determination of this mass-threshold is mandatory in order to understand how GCs form. We analyzed 9 RGB stars belonging to the cluster Ruprecht 106. Targets were observed with the UVES@VLT2 spectrograph. Spectra cover a wide range and allowed us to measure abundances for light (O,Na,Mg,Al), alpha (Si,Ca,Ti), iron-peak (Sc,V,Cr,Mn,Fe,Co,Ni,Cu,Zn) and neutron-capture (Y,Zr,Ba,La,Ce,Pr,Nd,Sm,Eu,Dy,Pb) elements. Based on these abundances we show that Ruprecht 106 is the first convincing example of a single population GC (i.e. a true simple stellar population), although the sample is relatively small. This result is supported also by an independent photometric test and by the HB morphology and the dynamical state. It is old (~12 Gyrs) and, at odds with other GCs, has no alpha-enhancement. The material it formed from was contaminated by both s- and r- process elements. The abundance pattern points toward an extragalactic origin. Its present day mass (M=10^4.83 Msun) can be assumed as a strong lower limit for the initial mass threshold below which no second generation is formed. Clearly, its initial mass must have been significantly greater but we have no current constraints on the amount of mass loss during its evolution.
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