Library preparation converts DNA or RNA molecules into sequencing-ready fragments that preserve biological complexity and ensure accurate representation across the genome or transcriptome. During this process, nucleic acids are fragmented, repaired, and ligated to indexed adapters that enable both amplification and sample multiplexing. The choice of chemistry—whether PCR-free for variant accuracy, ribosomal-depleted for transcriptome breadth, or bisulfite/enzymatic conversion for methylation profiling—directly influences the quality and interpretability of downstream data. GTAC@MGI tailors each library preparation to sample type, input quantity, and study goals, applying rigorous QC to minimize bias and maximize discovery potential across Illumina sequencing platforms.