An object of the invention is to provide a dephosphorylation enzyme that regulates cardiomyocyte differentiation, dominant negative enzyme thereof, a gene encoding the enzyme protein and use thereof. A protein or the like consisting of any one of the following amino acid sequences (A) to (C) is used: (A) the amino acid sequence of SEQ ID NO:2； (B) an amino acid sequence wherein one or several amino acids except for cysteine at position 138 are deleted, substituted or added in the amino acid sequence of SEQ ID NO:2, wherein a protein consisting of the amino acid sequence has a dual specificity phosphatase activity； and (C) an amino acid sequence having at least 60% or more homology to the amino acid sequence of SEQ ID NO:2 and having cysteine at position 138, wherein a protein consisting of the amino acid sequence has a dual specificity phosphatase activity.
