Illumina cluster detection algorithms are optimized around a balanced representation of A, T, G, and C nucleotides. Any divergence from equal base distribution will negatively influence the amount and quality of sequencing data produced. To increase the library nucleotide balance a spike-in of 20% PhiX will be used for samples with low diversity or unbalanced base composition (e.g., amplicons, bisulfite converted samples). The extent to which the negative impact of the unbalanced base composition will be reduced by spiking the PhiX control depends on individual sample and sequence characteristics. For more information please refer to the Illumina website.