The present invention provides a transgenic non-human mammal which lacks a functional Act1 gene referred to herein as a "transgenic non-human Act1 knockout mammal" or a "Act1 knockout mammal". In a particular embodiment, the genome of the Act1 knockout mammal comprises at least one non-functional allele for the endogenous Act1 gene. Thus, the invention provides a source of cells (for example, tissue, cells, cellular extracts, organelles) and transgenic non-human mammals useful for studying Act1. Further aspects of the invention provide methods for producing transgenic non-human Act1 knockout mammals and transgenic non-human Act1 deficient mammals produced by the methods; targeting vectors for use in producing Act1 deficient mammals; methods for the identification of agents (for example, diagnostic or therapeutic agents) which inhibit or mimic Act1 activity; and methods of identifying agents that can be used to treat and/or prevent a disease or condition associated with a defect in Act1 (e.g., systemic lupus erythematosus (SLE); Sjogren's syndrome, cancer).