Inhibitor subunit of the major nuclear protein phosphatase-1 (PP-1). It has RNA-binding activity but does not cleave RNA and may target PP-1 to RNA-associated substrates. May also be involved in pre-mRNA splicing. Binds DNA and might act as a transcriptional repressor. Essential for cell proliferation and early embryonic development. Interacts with phosphorylated CDC5L, SF3B1 and MELK. Part of the spliceosome. Interacts with PPP1CA, PPP1CB and PPP1CC (By similarity). Interacts with EED. Part of a complex consisting of PPP1R8, EED, HDAC2 and PP-1. Mainly, but not exclusively, nuclear. Has a basic N- and C-terminal and an acidic central domain. The FHA domain mediates interactions with threonine-phosphorylated MELK. May be inactivated by phosphorylation on Ser-199 or Ser-204. Mice display a retarded growth and embryonic lethality at E6.5, due to defects in proliferation rate.