PPP1R15B(Protein phosphatase 1, regulatory subunit 15b), also called CREP, promotes dephosphorylation of the transcription initiation factor EIF2-alpha through recruitment of protein phosphatase-1 (PP1) catalytic subunits. The PPP1R15B gene is mapped to chromosome 1q32.1 based on an alignment of thePPP1R15B sequence by Hartz(2010). Harding et al.(2009) obtained Ppp1r15b -/- mice at a mendelian ratio. However, Ppp1r15b -/- newborns were half the size of their wildtype littermates, were notably pale, and failed to nurse, and none survived the first day of postnatal life. Ppp1r15b -/- embryos that were also homozygous for an Eif2-alpha mutation that prevented Eif2-alpha phosphorylation were normalized, including elevated birth size and restored red blood cell count, compared with Ppp1r15b -/- embryos with wildtype Eif2-alpha.