Indicate whether each of the following descriptions applies to transformed cell lines (T), nontransformed cell lines (N), or both (B). Your answer would be a four-letter string composed of letters T, N, and B only, e.g. BBNN.
( ) They can be thawed and recultured after long-term storage in liquid nitrogen.
( ) They often grow only when attached to a surface, such as that of a culture dish.
( ) They proliferate to very high densities.
( ) They usually cause tumors when injected into immunocompromised animals.