The lac repressor is attached to the operator in the absence of lactose; it will only be removed from the operator if lactose attaches to it. Since there is a mutation that forbids this attachment, the lac repressor will always be on the operator. This will result in very low lac operon transcription, even in the presence of lactose.

Catabolite activator protein (CAP) will still bind upstream of the promoter, but RNA polymerase will be unable to access the operon due to the repressor's presence.

The primary transcript (immediately following transcription) is composed of multiple segments of RNA called introns and exons. While introns are always cleaved from the primary transcript, the exons may or may not stay on the primary transcript, which will eventually be translated. Since different numbers and orders of exons can arise from the same transcript, there are multiple proteins that can possibly be made from one primary transcript.

Intron and exon splicing, the addition of the 5' cap, and the addition of the poly A tail are all crucial post-transcriptional modifications in eukaryotes, but only the alternative splicing patterns result in variable transcripts. Eukaryotes do not follow the polycistronic model, in which one mRNA will code for multiple genes or proteins. The degeneracy of the genetic code refers to the multiple codons that will produce the same amino acids; this feature increases the variability of the DNA sequence, but does not affect the number of possible transcripts, genes, or proteins formed.

The poly A tail is an example of post-transcriptional processing. The poly A tail is attached to the end of the transcript, and protects it from being degraded by exonucleases. Exonucleases are essential in order to recycle the nucleobases used in mRNA, but can be harmful to RNA that has not yet been translated unless the poly A tail is present.

Both acetylation and methylation of histone tails modulate expression of target genes. Acetylation is always associated with active transcription while, in most cases, methylation is associated with repression of target genes. Acetylation generally loosens the DNA-histone association, allowing for transcription. In contrast, methylation generally tightens this association, blocking transcription proteins from binding to the DNA.

Hydroxylation is used to modify proteins and activate steroid hormones. Ubiquitination and sumoylation are post-translation protein modifications.

Western blots are used to measure the relative abundance of proteins. While there is a correlation between the amount of mRNA and protein, using a western blot to measure mRNA would be inconclusive due to the variability of protein half life.

Northern blots are used to run mRNA samples on gels, DNA microarrys give the expression levels of certain genes, and rtPCR is used to detect RNA expression levels. Any of these methods could provide the relative abundance of a particular mRNA.

Although each splice variant contains exons 4 and 7, PCR products between these exons in variants A and B would be too large to be detected using real time PCR; therefore, amplifying that region would conclusively identify the presence of variant C in HeLa cells. If variant C is not expressed in HeLa cells, one would expect a negative (or unamplified) result using real time PCR.

Variant A expression: 1, 2, 3, 4, 5, 6, 7

Variant B expression: 1, 3, 4, 6, 7

Variant C expression: 1, 2, 4, 7

By amplifying exons 5 and 6, and looking for a negative result, we can identify the presence of variant C.