 Homomorphism group of elliptic curves

The group ${\mathrm{Hom}}_K(E_1,E_2)$ consists of morphisms of elliptic curves defined over $K$ that preserve the group operation, with pointwise addition. Its zero element is the constant map to the identity. Every nonzero element is an isogeny of elliptic curves: its image is a closed connected subgroup of the target, so nonconstancy makes it the whole curve, and a nonconstant morphism of smooth projective curves is finite. The degree of an isogeny extends to this group by assigning degree zero to the zero homomorphism.