Ciliopathies are life-threatening human diseases caused by defective cilia. They 
can often be traced back to mutations of genes encoding transition zone (TZ) 
proteins demonstrating that the understanding of TZ organisation is of paramount 
importance. The TZ consists of multimeric protein modules that are subject to a 
stringent assembly hierarchy. Previous reports place Rpgrip1l at the top of the 
TZ assembly hierarchy in Caenorhabditis elegans By performing quantitative 
immunofluorescence studies in RPGRIP1L-/- mouse embryos and human embryonic 
cells, we recognise a different situation in vertebrates in which Rpgrip1l 
deficiency affects TZ assembly in a cell type-specific manner. In cell types in 
which the loss of Rpgrip1l alone does not affect all modules, additional 
truncation or removal of vertebrate-specific Rpgrip1 results in an impairment of 
all modules. Consequently, Rpgrip1l and Rpgrip1 synergistically ensure the TZ 
composition in several vertebrate cell types, revealing a higher complexity of 
TZ assembly in vertebrates than in invertebrates.