**10. Linkage Disequilibrium among** *CYP4* **Genes**

Five *CYP4* genes, *CYP4A22, CYP4A11, CYP4B1, CYP4X1,* and *CYP4Z1*, are located on chromosome 1 [4]. A number of studies based on next generation sequencing tools and a 1000-genome project have identified SNPs in these genes. However, their functional roles, clinical relationships, and linkage disequilibrium (LD) are poorly characterized. From the 1000-genome database, a total of 14 coding SNPs with > 5% global minimum allele frequency were identified for *CYP4A22, CYP4A11*, and *CYP4B1*, and this LD block was analyzed (Figure 1A). Ethnically distinct populations exhibited differing LD blocks and haplotype structures. No strong LD was found among these three *CYP4* genes that are clustered on chromosome 1. Six *CYP4F* genes, including *CYP4F2, CYP4F3, CYP4F8, CYP5F11, CYP4F12*, and *CYP4F22*, are located on the same chromatid of chromosome 19 [4]. Using the same method, coding variants with > 5% global frequency were selected from a 1000-genome database and their haplotypes and LD were analyzed (Figure 1B). As illustrated in Figure 1A, ethnically distinct groups showed differing frequencies and LD structures. An LD block covering more than one *CYP4* gene was not observed for *CYP4F* genes in coding variant analysis. Instead, a strong linkage was found between *CYP4F2* (rs2074900) and *CYP4F11* (rs8104361) in a Western European population. Since *CYP4* genes on the same chromosome with highly similar DNA structures can act as a linkage unit or as independent genes, further linkage analysis using more validated SNPs over all regions of *CYP4* genes is needed to improve the current knowledge of *CYP4* genetics.

**Figure 1.** *Cont.*

**Figure 1.** Linkage disequilibrium (LD) plots of *CYP4* genetic variants in African, Caucasian, and Asian populations. Populations in Yoruba, Utah, and Beijing represent African, Caucasian, and Asian populations, respectively. The coding single nucleotide polymorphisms (SNPs) with a minor allele frequency (MAF) of 0.05 or greater in the 1000 genome data base were selected to avoid estimation errors in linkage analysis. (**A**) LD structures of *CYP4A11*, *CYP4A22*, and *CYP4B1* with common coding SNPs. *CYP4A11*, *CYP4A22*, and *CYP4B1* are clustered on chromosome 1. The SNPs, shown from left to right within the figure, are as follows: rs4646487, rs2297810, rs4646491, rs2297809, rs1126743, rs1126742, rs12564525, rs2056900, rs2056899, rs113777592, rs10789501, rs149718343, rs2405599, and rs4926600. (**B**) LD structures of *CYP4F2*, *CYP4F3*, *CYP4F11*, and *CYP4F12* using common coding SNPs. *CYP4F2*, *CYP4F3*, *CYP4F11*, and *CYP4F12* are clustered on chromosome 19. The SNPs, shown from left to right within the figure, are as follows: rs1805040, rs7254013, rs16995376, rs16995378, rs609636, rs609290, rs2285888, rs593818, rs3093200, rs2108622, rs2074900, rs3093105, rs1060463, and rs8104361. The numbers in squares refer to pairwise LD values, measured as D' (coefficient of linkage disequilibrium). Red depicts a significant linkage between a pair of SNPs. Numbers inside squares indicate the D' value multiplied by 100.
