Normalized fragile codon content (NFCU) (controlling for GC-content and amino-acid usage) of multi-exon genes and single-exon genes in the human, mouse, and fly genomes.

Single-exon genes were first treated as a single group to determine the impact of EJC-dependent NMD on fragile codon content and subsequently subdivided into histone and non-histone genes to consider the impact of PABP-dependent NMD on fragile codon content. NFCU, median (first and third quartiles) of fragile codon content normalized for GC-content and amino acid usage; P-value, significance of the comparison with multi-exon genes determined by two-sided Wilcoxon-rank sum test that tests for a linear shift in distribution locations; Ratio, median NFCU relative to the median NFCU for multi-exon genes.我们首先将单外显子基因（Single-exon genes）设为单一组别，以探究外显子连接复合物（Exon Junction Complex, EJC）依赖的无义介导的mRNA降解（Nonsense-mediated mRNA decay, NMD）对脆性密码子含量的影响；随后将该组别细分为组蛋白基因与非组蛋白基因，以评估多聚腺苷酸结合蛋白（Poly(A)-binding protein, PABP）依赖的NMD对脆性密码子含量的作用。标准化脆性密码子含量（Normalized Fragile Codon Content, NFCU）指经GC含量与氨基酸使用情况标准化后的脆性密码子含量的中位数（第一四分位数与第三四分位数）；P值为通过双侧Wilcoxon秩和检验（two-sided Wilcoxon-rank sum test）得到的与多外显子基因（Multi-exon genes）进行比较的显著性水平，该检验用于检测分布位置的线性偏移；比值（Ratio）为目标组NFCU中位数与多外显子基因NFCU中位数的相对比值。