The invention relates to methods of modifying cells of corneal tissue to express an active agent, to modified corneal tissue, to vectors utilised in such methods and to methods of xeno- and allo-transplantation utilising the modified corneal tissue. The method of modifying cells of corneal tissue to express an active agent involves exposing harvested corneal tissue to an effective concentration for transfection of an expression vector which comprises a nucleotide sequence encoding for the active agent for a period sufficient to allow infection, such that cells of the corneal tissue will express the active agent.