Dephosphorylates CDKN1B at 'Thr-187', thus removing a signal for proteasomal degradation. H2O + O-phospho-L-seryl-[protein] = L-seryl-[protein] + phosphate H2O + O-phospho-L-threonyl-[protein] = L-threonyl-[protein] + phosphate May act as a suppressor of trastuzumab resistance. Belongs to the PP2C family. A report observed N-glycosylation at Asn-354 (PubMed:19139490). However, as the protein is not predicted to localize in an extracellular compartment of the cell, additional evidence is required to confirm this result.