Expression of foreign proteins in both cultured and intact mammalian cells has become essential to evaluating cellular biology and physiology including the proteins involved in the control of eicosanoid production. Many transient expression vectors are commercially available and are in widespread use. However, some cell types, such as the renal mesangial cell, have been difficult to transform with these vectors and require a different strategy to obtain stable expression of the exogenous gene. Viral vectors, including retroviruses, adenoviruses, and the Sinbis virus have been developed over the last decade for foreign gene expression in mammalian cells. This chapter will discuss the use of a replication-incompetent retroviral vector in cell culture to express an exogenous protein.