In order to obtain single crystal structures of the three variants of the GNCA4 β-lactamases of interest to this work, we followed a similar protocol already described elsewhere. The three proteins were subject to crystallization assays by the capillary counterdiffusion techniques and by vapor-diffusion (VD) using the hanging drop set-up. We prepared a small screening around the known successful conditions previously used to crystallized GNCA4 and GNCA4-WT variants. Ιn brief, for counterdiffusion experiments, each protein was concentrated to 23-25 mg•ml -1 , loaded in capillaries of 0.3 mm inner diameter and confronted to 5 M sodium formate in the pH range of 4.0 to 9.0. For VD 1 µL of protein solution was mixed with the reservoir, in a 1:1 ratio, and equilibrated against 500 µL of each precipitant cocktail (4 M sodium formate in the pH range of 4.0 to 9.0). The best-looking crystals of GNCA4-2 & GNCA4-12 were obtained at pH 4.0 using the counterdiffusion technique, while in the case of GNCA4-19, they grew at pH 7.0 in hanging drop.