Linkage disequilibrium
	necessary for association studies highly variable throughout the genome as a function of distance Kruglyak: use d2 as a marker = squared difference of marker frequency on normal and variant chromosomes sample size µ1/ d2, so we want d2 > 0.3, say. simulation suggests 3kb distance, on the average, corresponds to d2 = 0.4 (1 Mb = 1 cM) this is too conservative in some parts of the genome.