In which isochores are Alu elements enriched?
[' the vast majority of Alu sequences were shown to have the highest density in GC-rich isochores', 'Alu sequences being unstable in the GC-poor isochores but stable in the compositionally matching GC-rich isochores,', 'Alu retrotransposons do not show a homogeneous distribution over the human genome but have a higher density in GC-rich (H) than in AT-rich (L) isochores', 'This result, together with the known higher selective disadvantage of recombination products in H isochores, points to Alu-Alu recombination as the main agent provoking the density shift of Alus toward the GC-rich parts of the genome', 'whereas GC-rich Alus are mostly present in GC-rich isochores', 'the present results on Alu and LINE stability/exclusion predict significant losses of Alu DNA from the GC-poor isochores during evolution, a phenomenon apparently due to negative selection against sequences that differ from the isochore composition', 'The frequency of Alu sequences also increases with increasing GC, but attains a maximum in H2 isochores, in agreement with previous experimental data', 'The CpG levels of both Alus and CpG islands increase with their GC levels', 'characterized as a GC-rich isochore enriched for CpG islands, genes, and Alu-like repeats', 'Microsatellites and SINES (Alu, B1, B2) are found at roughly equal frequencies in introns from all isochore classes', 'The results indicate that the short repeats of the B1 family of mouse and of the Alu I family of man are most frequent in the heavy components, whereas the long repeats of the BamHI family of mouse and of the Kpn I family of man are mainly present in the two light components.']
['Alu elements are enriched in high GC% isochores due to reduced Alu loss by recombination in these regions. The frequency of Alu sequences increases with increasing GC, but attains a maximum in H2 isochores.']
['H2', 'H3']