Distribution of the Def_HI parameter of galaxies in the Coma supercluster as a function of the local galaxy density δ_1,1000. Filled dots represent HI data from ALFALFA, empty symbols derive from deeper, pointed HI measurements of optically selected galaxies. Clearly ALFALFA is not sufficiently deep to detect deficient objects, except for few massive, face-on galaxies (the only point above Def_HI = 1 is NGC 4921, a face-on giant Sb in the Coma cluster). Deeper, pointed HI observations reveal the expected increase of the mean HI deficiency with increasing local galaxy density (red symbols).