Human SARS coronavirus 1 (SARS-CoV-1) non-structural protein 15 (nsp15) contains the LXCXE/D motif characteristic of proteins that bind to the retinoblastoma protein RB1. Binding to human RB1 increases the endonuclease activity of nsp15 but is not required for it. RB1 bound to nsp15 is retained in the cytosol. Interaction of nsp15 with RB1 likely affects the cell cycle of infected cells and probably modulates cytotoxicity of SARS-CoV-1 (Bhardwaj et al. 2012).