Abstract: A method is provided for selecting an improved retroviral genome having an improved packaging efficiency which method comprises: a) introducing one or more random mutations into a retroviral genome comprising a packaging signal; b) introducing the mutagenised retroviral genome into a host cell expressing viral polypeptides required for packaging the retroviral genome; c) determining whether retroviral packaging efficiency in the cell is improved as compared with a retroviral genome comprising a non-mutated packaging signal; d) selecting a viral genome which has improved packaging efficiency.

Abstract: A method is provided for enhancing the production of an infectious retrovirus comprising an envelope polypeptide in a producer cell which method comprises inhibiting the expression or activity in the producer cell of an endogenous receptor which is capable of binding to the envelope polypeptide of said retroviruses.