TAC1 and cancer: The crystal structure of a complex of NKA in the phosphoenzyme (E2P) form with digoxin, a representative cardiac glycoside, showed that the 14β and 12β hydroxy groups, the 17β-lactone unit, and the first proximal digitoxose moiety of digoxin all contribute substantially to its interaction with NKA and to its cancer cell line cytotoxicity observed [18,19,20].