Phospholamban (PLN) is a regulatory protein found primarily in cardiac muscle cells. It plays a key role in controlling heart muscle contraction and relaxation by modulating the activity of the sarcoplasmic reticulum calcium pump (SERCA). At rest, PLN inhibits SERCA to regulate calcium levels, helping maintain calcium balance in heart cells. When stimulated, PLN is phosphorylated, which lifts this inhibition and boosts calcium uptake, supporting efficient cardiac muscle relaxation and overall heart function.