  To understand globular clusters (GCs) we need to comprehend how their
formation process was able to produce their abundance distribution of light
elements. In particular, we seek to figure out which stars imprinted the
peculiar chemical signature of GCs. One of the best ways is to study the
light-element anti-correlations in a large sample of GCs that are analysed
homogeneously. As part of our spectroscopic survey of GCs with FLAMES, we
present here the results of our study of about 30 red giant member stars in the
low-mass, low-metallicity Milky Way cluster NGC 6535. We measured the
metallicity (finding [Fe/H]=-1.95, rms=0.04 dex in our homogeneous scale) and
other elements of the cluster and, in particular, we concentrate here on O and
Na abundances. These elements define the normal Na-O anti-correlation of
classical GCs, making NGC 6535 perhaps the lowest mass cluster with a confirmed
presence of multiple populations. We updated the census of Galactic and
extragalactic GCs for which a statement on the presence or absence of multiple
populations can be made on the basis of high-resolution spectroscopy
preferentially, or photometry and low-resolution spectroscopy otherwise; we
also discuss the importance of mass and age of the clusters as factors for
multiple populations.
