Is it ensure, insure, or assure?
heterologous het·er·ol·o·gous (hět'ə-rŏl'ə-gəs)
Of or relating to cytologic or histological elements not normally occurring in a designated part of the body.
Derived from a different species, as a graft or transplant.
Immunologically related but not identical. Used of certain cells and antiserums.