Functions as an integral cofactor in cell pH regulation by controlling plasma membrane-type Na(+)/H(+) exchange activity. Promotes the induction of hematopoietic stem cell differentiation toward megakaryocytic lineage. Essential for the coupling of ERK cascade activation with the expression of ETS family genes in megakaryocytic differentiation. Also involved in granulocytic differentiation in a ERK-dependent manner. Inhibits the phosphatase activity of calcineurin (By similarity). Monomer (By similarity). Homodimer (By similarity). Binds calcium via its EF-hands. Calcium-binding mediates a conformational change. Can also bind magnesium (By similarity). Belongs to the calcineurin regulatory subunit family. CHP subfamily.