Tetramers of alpha- and beta-spectrin heterodimers, linked by intermediary proteins to transmembrane proteins, stabilize the red blood cell cytoskeleton. Deficiencies of either alpha- or beta-spectrin can result in severe hereditary spherocytosis (HS) or hereditary elliptocytosis (HE) in mice and humans. Four mouse mutations, sph, sph(Dem), sph(2BC), and sph(J), affect the erythroid alpha-spectrin gene, Spna1, on chromosome 1 and cause severe HS and HE. Here we describe the molecular alterations in alpha-spectrin and their consequences in sph(2BC)/sph(2BC) and sph(J)/sph(J) erythrocytes. A splicing mutation, sph(2BC) initiates the skipping of exon 41 and premature protein termination before the site required for dimerization of alpha-spectrin with beta-spectrin. A nonsense mutation in exon 52, sph(J) eliminates the COOH-terminal 13 amino acids. Both defects result in instability of the red cell membrane and loss of membrane surface area. In sph(2BC)/sph(2BC), barely perceptible levels of messenger RNA and consequent decreased synthesis of alpha-spectrin protein are primarily responsible for the resultant hemolysis. By contrast, sph(J)/sph(J) mice synthesize the truncated alpha-spectrin in which the 13-terminal amino acids are deleted at higher levels than normal, but they cannot retain this mutant protein in the cytoskeleton. The sph(J) deletion is near the 4.1/actin-binding region at the junctional complex providing new evidence that this 13-amino acid segment at the COOH-terminus of alpha-spectrin is crucial to the stability of the junctional complex.