TABLE 2.

Pharmacogenes with the associated challenges that render them difficult to genotype.

Gene	Challenge(s)	Reference
CYP2D6	–Structural variants and gene rearrangements	Taylor et al. (2020)
	–Pseudogenes	PharmVar structural variations CYP2D6
	–Copy Number Variations
	–Presence of novel variants
	–Highly polymorphic region
	–Substrate-specific effects of some alleles
UGT1A1	–Rare population-specific variants	Barbarino et al. (2014)
	–Variants in non-coding parts of the gene	Marques and Ikediobi, 2010
	–Independent haplotypes with less linkage disequilibrium
VKORC1	–Important variants in non-coding parts of the gene	Saminathan et al. (2010)
		Owen et al., 2010
HLA	–Rare population-specific variants	Illing et al. (2017)
	–Highly polymorphic regions	Klasberg et al., 2019
SLC6A4	–Rare population-specific variants	Lam (2013)