Phospholamban (PLN) is a regulatory protein primarily found in cardiac muscle cells. It plays a crucial role in controlling the contraction and relaxation of heart muscle 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 activated, PLN is phosphorylated, which removes this inhibition and increases calcium uptake, supporting efficient cardiac muscle relaxation and overall heart function.