The human genome is divided into isochores, large stretches (>>300 kb) of 
genomic DNA with more or less consistent GC content. Mutational/neutralist and 
selectionist models have been put forward to explain their existence. A major 
criticism of the mutational models is that they cannot account for the higher GC 
content at fourfold-redundant silent sites within exons (GC4) than in flanking 
introns (GCi). Indeed, it has been asserted that it is hard to envisage a 
mutational bias explanation, as it is difficult to see how repair enzymes might 
act differently in exons and their flanking introns. However, this rejection, we 
note, ignores the effects of transposable elements (TEs), which are a major 
component of introns and tend to cause them to have a GC content different from 
(usually lower than) that dictated by point mutational processes alone. As TEs 
tend not to insert at the extremities of introns, this model predicts that GC 
content at the extremities of introns should be more like that at GC4 than are 
the intronic interiors. This we show to be true. The model also correctly 
predicts that small introns should have a composition more like that at GC4 than 
large introns. We conclude that the logic of the previous rejection of 
neutralist models is unsafe.