Cells capable of at least, in part, complementing adenovirus an adenovirus defective in E2A function. Such cells include a nucleic acid-encoding adenovirus E2A or a functional part, derivative, temperature-sensitive mutation and/or analogue thereof, integrated into the cell's genome. Methods for producing an adenovirus particle/vector with a functional deletion of E2A are also disclosed. Such methods involve providing a cell with the functionally deleted adenovirus vector, culturing the cell, and harvesting viral particles. The functional deletion may comprise a deletion in E2A. The nucleic acid-encoding E2A in the cell's genome may lack sequence overlap with the vector, preventing formation of a replication-competent adenovirus or restoration of E2A function. The adenovirus vector may further include a functional deletion in the E1-region. Methods are disclosed for providing cells of an individual with a nucleic acid of interest, without risk of administering simultaneously a replication-competent adenovirus vector, comprising administering the individual an adenovirus vector described herein.