SEC23B gene encodes an essential component of the coat protein complex II (COPII)-coated vesicles. Mutations in this gene cause the vast majority the congenital dyserythropoietic anemia Type II (CDA II), a rare disorder resulting from impaired erythropoiesis. Here, we investigated 28 CDA II patients from 21 unrelated families enrolled in the CDA II International Registry. Overall, we found 19 novel variants [c.2270 A>C p.H757P; c.2149−2 A>G; c.1109+1 G>A; c.387(delG) p.L129LfsX26; c.1858 A>G p.M620V; c.1832 G>C p.R611P; c.1735 T>A p.Y579N; c.1254 T>G p.I418M; c.1015 C>T p.R339X; c.1603 C>T p.R535X; c.1654 C>T p.L552F; c.1307 C>T p.S436L; c.279+3 A>G; c. 2150(delC) p.A717VfsX7; c.1733 T>C p.L578P; c.1109+5 G>A; c.221+31 A>G; c.367 C>T p.R123X; c.1857_1859delCAT; p.I619del] in the homozygous or the compound heterozygous state. Homozygosity or compound heterozygosity for two nonsense mutations was never found. In four cases the sequencing analysis has failed to find two mutations. To discuss the putative functional consequences of missense mutations, computational analysis and sequence alignment were performed. Our data underscore the high allelic heterogeneity of CDA II, as the most of SEC23B variations are inherited as private mutations. In this mutation update, we also provided a tool to improve and facilitate the molecular diagnosis of CDA II by defining the frequency of mutations in each exon. Am. J. Hematol., 2010. © 2010 Wiley-Liss, Inc.