A. Schematic representation of human FAM111A domains and mutations that compromise FAM111A functions. The FAM111A N-terminus contains a functional PCNA interaction motif (PIP box; Alabert et al., 2014()). The FAM111A C-terminal trypsin-like protease domain contains a typical H385-D439-S541 catalytic triad Unger et al., 2013(), and binds the C-terminus of the SV40 Large T antigen Fine et al., 2012Tarnita et al., 2019(, ). We used an S541A mutation to disrupt the catalytic triad and inactivate FAM111A's trypsin-like protease activity. Finally, the KCS2 and OCS patient mutations are indicated in red boxes. See text and Unger et al., 2013() for phenotypic details. 