The Sprn gene encodes Shadoo (Sho), a glycoprotein with biochemical properties similar to the unstructured region of cellular prion protein (PrP^C). Sho has been considered a candidate for the hypothetical π protein that supplies a PrP^C-like function to maintain the viability of Prnp^0/0 mice lacking the PrP^C protein. To understand these relationships more clearly we probed the cell biology of Sho and created knockout mice. Besides full-length and a “C1” C-terminal fragment, we describe a 6-kDa N-terminal Sho neuropeptide, “N1,” which is present in membrane-enriched subcellular fractions of wild-type mice. Sprn null alleles were produced that delete all protein coding sequences yet spare the Mtg1 gene transcription unit that overlaps the Sprn 3′ UTR; the resulting mice bred to homozygosity were viable and fertile, although Sprn^0/0 mice maintained in two genetic backgrounds weighed less than wild-type mice. Lack of Sho protein did not affect prion incubation time. Contrasting with lethality reported for knockdown of expression in Prnp^0/0 embryos using lentiviruses targeted against the Sprn 3′ UTR, we established that double-knockout mice deficient in both Sho and PrP^C are fertile and viable up to 690 d of age. Our data reduce the impetus for equating Sho with the notional π protein and are not readily reconciled with hypotheses wherein expression of PrP^C and Sho are both required for completion of embryogenesis. Alternatively, and in accord with some reports for PrP^C, we infer that Sho’s activity will prove germane to the maintenance of neuronal viability in postnatal life.