cilia microtubules undergo ongoing turnover ( Stephens, 1997 ),
whereas in others, the axonemal microtubules appear to be much
less dynamic. Therefore, the degree to which the machinery
of ciliogenesis, including IFT (see Scholey on p. 23 of this
issue), is required in these different types of cilia will clearly
differ, with more dynamic cilia requiring a higher effi cacy of
continual assembly. Such cilia would be the fi rst to go in a disease
mutation that partially reduced IFT. Therefore, hypomorphic
alleles of ciliogenesis genes might cause defects in only a subset
of cilia, leading to an overall phenotype that differs from
that of a null mutant.