PAGE and staining for PK activity in extracts of fetal livers or RBCs. Fetal livers were obtained from 16-day embryos of CBA-+/+ or -Pk-1^slc/Pk-1^slc mice; RBCs from 1-day-old and 7-day-old CBA-+/+ or -Pk-1^slc/Pk-1^slc mice. M2, M2-PK; R/L, R- or L-PK.