The present invention provides an avian cell in which the expression or activity of one or more of the following genes, or a homologue thereof: Chicken IFITM1 (SEQ ID No. 1); Chicken IFITM2 (SEQ ID No. 2) and Chicken IFITM3 (SEQ ID No. 3) is reduced. The invention also provides methods for passaging viruses in avian cells, embryos and/or avian cell lines which have reduced expression of one or more IFITM genes and methods which involve investigating the sequence of one or more of the following genes, or a homologue thereof: Chicken IFITM1 (SEQ ID No. 1); Chicken IFITM2 (SEQ ID No. 2) and Chicken IFITM3 (SEQ ID No. 3).