Ageneticist introduces a transgene into yeast cells and isolates five independent cell lines in which the transgene has integrated into the yeast genome. in four of the lines, the transgene is expressed strongly, but in the fifth there is no expression at all. which is a likely explanation for the lack of transgene expression in the fifth cell line?

I think the best explanation would be that the transgene integrated into a heterochromatic region of the genome. Heterochromatin are chromosomal regions of the genome that lack high numbers of genes while chromosomal regions contain high concentrations of transcribed genes and make part of the relaxed euchromatin. A transgene is a genetic material that has been transferred naturally, or by any of a number of genetic engineering techniques from one organism to another.