Table 1 The properties required for development of an ideal vector system and those that need to be avoided.

Stable high titre vectors

Vector degradation

Simple and reproducible production

Replication competent virus

Unlimited packaging capacity

Expression of undesirable viral proteins

Efficient gene transfer to the target cells

Germline gene transmission

Controlled genomic integration

Insertional mutagenesis

Regulated normal levels of expression

Inappropriate toxic expression

Ability to repeat delivery if needed

Severe immune response against vector system

Long-term expression

Immune response against transgene products