Giardia lamblia encystation is crucial for disease transmission and parasite survival outside the host. We previously demonstrated that the enzyme arginine deiminase (ADI) translocate to the nuclei where it acts as peptidyl-arginine deiminase on histones late during encystation, leading to the downregulation of the expression of cyst wall protein 2. The translocation to the nuclei is mainly regulated by the sumoylation of ADI since when the enzyme had mutations in the SUMOylation site, the translocation to the nuclei is reduced, and the number of cysts produced were similar to wild type trophozoites. Proteins destined for transport in the nucleus contain amino acid targeting sequences called nuclear localization signals (NLSs). ADI contain a conserved bipartite NLS and a monopartite consensus sequence. Substitutions made in the first part of the monopartite or the bipartite NLSs led to accumulation of ADI in the nuclei, indicating normal entry of ADI into the nuclei. Unexpectedly, ADI in which two hydrophobic residues (V and M) were mutated to alanine residues failed to enter the nucleus. To conclude, we show that ADI is a nucleocytoplasmatic shuttling protein. Its entry into the nuclei depends on its sumoylation and on the presence of particular residues on its NLS. Also, its function inside them is the citrullination of histones that probably causes genes in the CWP family to turn off as an essential requirement to successfully complete the encystations of the parasite.