TABLE 2.

Specific Predictions and the Fit of the Three Main Models to These Data^a

Experimental question	Cilia model predicts	Cytoplasmic model predicts	Chromatid segregation model predicts	Experimental result
Should mutation of kinesin, dynein, MTOC, and PCP proteins randomize LR?	YES	YES	YES	YES
Should viscosity changes at the node randomize LR?	YES	no, unless cilia amplify	no, unless cilia amplify	YES
Should chick embryos have asymmetric gene expression before node forms?	NO	YES	YES	YES
Should frog embryos establish asymmetric gradients long before cilia form?	NO	YES	YES	YES
Should disruption of cytoskeleton or of physiological asymmetries only during the first couple of cleavages randomize LR?	NO	YES	YES	YES
Should chick node receive LR information from lateral tissue?	NO	YES	NO	YES
Should embryos (human & newt) randomize if split at the two-cell stage?	NO	YES	YES	YESb
Should the brains of primary ciliary dyskinesia patients have normal laterality?	NO	YES	YES	YES
Should any mutants exist with abnormal ciliary flow but normal LR asymmetry?	NO	YES	YES	YES
Should animals and plants with no cilia and no node be able to establish asymmetry using some of the same molecules as ciliated vertebrates?	NO	YES	YES	YES
Should organizers induced past the first few cleavages be randomized?	NO	YES	YES	YES

^aThe three main models make distinct predictions as to the outcomes of a number of experimental questions. Most work on cilia has focused on mutations of ciliary genes, which does not distinguish between these models (since many of these same components also play important roles in intracellular polarity determination and are present in non-ciliated cells). However, a variety of extant data do distinguish between the models; with the exception of the two studies that have shown that fluid flow per se can affect asymmetry (clearly supporting the ciliary model), the majority of the data support the early models, not the ciliary model. Green = consistent and predicted by model. Yellow = not predicted by model but does not contradict it. Red = contradicts the predictions of the model; LR, left–right.

^bHuman twins show subtle bookending (opposite asymmetry of unilateral defects), not real laterality disturbances.