POM121 and gp210 were, until this point, the only known membrane-integral 
nucleoporins (Nups) of vertebrates and, thus, the only candidate anchors for 
nuclear pore complexes (NPCs) within the nuclear membrane. In an accompanying 
study (Stavru et al.), we provided evidence that NPCs can exist independently of 
POM121 and gp210, and we predicted that vertebrate NPCs contain additional 
membrane-integral constituents. We identify such an additional membrane protein 
in the NPCs of mammals, frogs, insects, and nematodes as the orthologue to yeast 
Ndc1p/Cut11p. Human NDC1 (hNDC1) likely possesses six transmembrane segments, 
and it is located at the nuclear pore wall. Depletion of hNDC1 from human HeLa 
cells interferes with the assembly of phenylalanine-glycine repeat Nups into 
NPCs. The loss of NDC1 function in Caenorhabditis elegans also causes severe NPC 
defects and very high larval and embryonic mortality. However, it is not 
ultimately lethal. Instead, homozygous NDC1-deficient worms can be propagated. 
This indicates that none of the membrane-integral Nups is universally essential 
for NPC assembly, and suggests that NPC biogenesis is an extremely 
fault-tolerant process.