Genomic libraries are prepared from random fragments of genomic DNA.

然而，基因组文库作为寻找一种特定基因的方法效率并不高，尤其对于含有大量非编码DNA的真核基因组中。

However, genomic libraries may be an inefficient method of finding a gene, particularly in large eukaryotic genomes, where much of the DNA is noncoding.

替代的方法是利用已知表达目的基因的细胞或组织来源的mRNA所构建的文库。