The first use of this strain designation was UM-HET3
Miller, R. A., C. Chrisp, and A. Galecki. 1997. CD4 memory T cell levels predict lifespan in genetically heterogeneous mice. FASEB Journal 11:775-783
UM-HET3 mice are neither an F1 nor an F2 hybrid. (An F2 hybrid is made by crossing males and females of the same F1 hybrid stock.) The correct designation is a "four-way cross."
Publications from the NIA Interventions Testing Program
The first ITP paper also referred to these mice as UM-HET3, using this strain designation regardless of the site at which the mice were produced. See:
Miller, R. A., D. E. Harrison, C. M. Astle, R. A. Floyd, K. Flurkey, K. L. Hensley, M. A. Javors, C. Leeuwenburgh, J. F. Nelson, E. Ongini, N. L. Nadon, H. R. Warner, R. Strong. 2007. An aging interventions testing program: study design and interim report. Aging Cell 6: 565 - 575. [PMC: 17578509]