This study describes a dominant behavioral marker, rol-6(su1006), and an efficient microinjection procedure for DNA transformation in Caenorhabditis elegans. These tools were used to investigate the mechanism of DNA transformation in C. elegans. By injecting mixtures of genetically marked DNA molecules, the researchers showed that large extrachromosomal arrays assemble directly from the injected molecules, driven by homologous recombination. Appropriately placed double-strand breaks stimulated homologous recombination during array formation. The size of the assembled transgenic structures determines whether they are maintained extrachromosomally or lost. Low copy number extrachromosomal transformation can be achieved by adjusting the relative concentration of DNA molecules in the injection mixture. Integration of the injected DNA, though relatively rare, was reproducibly achieved when single-stranded oligonucleotide was co-injected with the double-stranded DNA. The study found that the assembly of heritable extrachromosomal elements depends on interactions between injected molecules. The structure of extrachromosomal arrays was analyzed, and it was shown that independent transforming elements assemble from the injected DNA and are inherited by separate progeny of the injected animal. The composition of arrays assembled from a mixture of genetically marked molecules was investigated, and it was found that shared regions of homology are important for driving the assembly of molecules into arrays. Homologous recombination during array formation was explored, and it was found that the length of homology affects the frequency of recombination. Double-strand breaks stimulated homologous recombination during array assembly, consistent with effects observed in other systems. The study also showed that extrachromosomal arrays contain numerous rearrangements of the injected DNA, and that the majority of these rearrangements occur within or between homologous molecules. Chromosomal integration was investigated, and it was found that DNA injected into the C. elegans gonad cytoplasm undergoes a transient period of reactivity resulting in the formation of large heritable extrachromosomal elements. The study also showed that the frequency of integration can be increased by co-injecting plasmid DNA with an excess of single-stranded oligonucleotide. The heritability of transgenic structures was found to depend on the size of the arrays, with larger arrays being more likely to be maintained extrachromosomally. The expression of transgenic sequences was found to be generally comparable with expression after chromosomal integration. The findings have practical applications for C. elegans transformation, allowing the recovery of transformed lines and the control of extrachromosomal array composition. The study highlights the importance of homologous recombination and double-strand breaks in the formation of extrachromosomal arrays and chromosomal integration in C. elegans. The results suggest that C. elegans is an ideal organism for studying gene function and regulation using molecular techniques.This study describes a dominant behavioral marker, rol-6(su1006), and an efficient microinjection procedure for DNA transformation in Caenorhabditis elegans. These tools were used to investigate the mechanism of DNA transformation in C. elegans. By injecting mixtures of genetically marked DNA molecules, the researchers showed that large extrachromosomal arrays assemble directly from the injected molecules, driven by homologous recombination. Appropriately placed double-strand breaks stimulated homologous recombination during array formation. The size of the assembled transgenic structures determines whether they are maintained extrachromosomally or lost. Low copy number extrachromosomal transformation can be achieved by adjusting the relative concentration of DNA molecules in the injection mixture. Integration of the injected DNA, though relatively rare, was reproducibly achieved when single-stranded oligonucleotide was co-injected with the double-stranded DNA. The study found that the assembly of heritable extrachromosomal elements depends on interactions between injected molecules. The structure of extrachromosomal arrays was analyzed, and it was shown that independent transforming elements assemble from the injected DNA and are inherited by separate progeny of the injected animal. The composition of arrays assembled from a mixture of genetically marked molecules was investigated, and it was found that shared regions of homology are important for driving the assembly of molecules into arrays. Homologous recombination during array formation was explored, and it was found that the length of homology affects the frequency of recombination. Double-strand breaks stimulated homologous recombination during array assembly, consistent with effects observed in other systems. The study also showed that extrachromosomal arrays contain numerous rearrangements of the injected DNA, and that the majority of these rearrangements occur within or between homologous molecules. Chromosomal integration was investigated, and it was found that DNA injected into the C. elegans gonad cytoplasm undergoes a transient period of reactivity resulting in the formation of large heritable extrachromosomal elements. The study also showed that the frequency of integration can be increased by co-injecting plasmid DNA with an excess of single-stranded oligonucleotide. The heritability of transgenic structures was found to depend on the size of the arrays, with larger arrays being more likely to be maintained extrachromosomally. The expression of transgenic sequences was found to be generally comparable with expression after chromosomal integration. The findings have practical applications for C. elegans transformation, allowing the recovery of transformed lines and the control of extrachromosomal array composition. The study highlights the importance of homologous recombination and double-strand breaks in the formation of extrachromosomal arrays and chromosomal integration in C. elegans. The results suggest that C. elegans is an ideal organism for studying gene function and regulation using molecular techniques.