- Previous methods for generating TIL require screening for anti-tumor specificity using gamma-interferon (IFN) production by the TIL. However, in vitro screening depends on autologous tumor reagents that are often unavailable; and gamma-IFN release in vitro may not be the best correlate to in vivo efficacy. Additionally, this method necessitates long in vitro culture times (44 days), and therefore reduces the clonal heterogeneity of TIL cultures, and results in TIL cultures with shorter telomere lengths and phenotypes that are skewed toward a more differentiated phenotype.