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FIGURE 4-1 Exogenous and endogenous factors contribute to variation in drug response.

expression of this crucial glucuronosyl transferase in liver. Finally, a common 68 base pair indel polymorphism in cystathionine b-synthase has been linked to folate levels.

A haplotype, defined as a series of linked polymorphisms on a chromosome, specifies the DNA sequence variation on one chromosome. For example, consider two SNPs in ABCB1 encoding the P-glycoprotein multidrug resistance protein: a T to A base pair substitution at position 3421 and a C to T change at position 3435. Possible haplotypes are T3421C3435, T3421T3435, A3421C3435, and A3421T3435. For any gene, individuals will have two haplotypes, one maternal and one paternal in origin, which may or may not be identical. Haplotypes are important because they are the functional unit of the gene. That is, a haplotype represents the constellation of variants that occur together for the gene on each chromosome. In some cases, this constellation of variants, rather than the individual variant or allele, may be functionally important. In others, however, a single mutation may be functionally important regardless of other variants within the haplotype(s).

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