We have analyzed the effects of linearizing vector DNA on the frequency and pathway of its recombination with the homologous chromosomal gene. The pSV2neo vector bearing a 4.3-kb fragment encoding the mouse immunoglobulin mu heavy chain constant (C mu) region was cut either at sites within the C mu segment or outside C mu and then transferred to hybridoma cells bearing a mutant mu gene. The frequency of recombinant cells producing normal mu was then measured. For most cut sites, whether in regions of homology or of nonhomology, linearization of the transferred DNA enhanced the recombination frequency between the vector and chromosomal mu genes. When the vector was either uncut or cut at SacI in the region of homology, G418-resistant mu m+ recombinants were found to have integrated the vector by a single reciprocal homologous crossover; the enzyme site (SacI) used for cutting was present in the recombinants. By contrast, when the vector had been linearized at PvuI or SfiI in the region of nonhomology, vector integration involved nonhomologous crossovers, either between transferred DNA molecules or between transferred and chromosomal DNA, and the vector cut sites were absent in these recombinants. Some recombinants were found to have an unaltered as well as recombinant mu gene, suggesting that the nonhomologous recombination process might have involved sister chromatids.

It has long been observed that human protein‐coding genes have a particular distribution of GC‐content: the 5′ end of these genes has high GC‐content while the 3′ end has low GC‐content. In 2012, it was proposed that this pattern of GC‐content could act as an mRNA identity feature that would lead to it being better recognized by the cellular machinery to promote its nuclear export. In contrast, junk RNA, which largely lacks this feature, would be retained in the nucleus and targeted for decay. Now two recent papers have provided evidence that GC‐content does promote the nuclear export of many mRNAs in human cells.