What does microsatellite instability indicate?

Microsatellite instability (MSI) is a condition characterized by changes in the length of microsatellite regions in the DNA due to the DNA mismatch repair system's failure. When this repair system is compromised, errors that occur during DNA replication are not corrected, leading to the expansion or contraction of these repetitive sequences. Thus, the correct answer highlights that microsatellite instability indicates the presence of an incorrect number of microsatellite repeats, which can be a significant marker for certain types of cancers, especially in relation to hereditary nonpolyposis colorectal cancer (Lynch syndrome).

In contrast, the other options are less relevant. The absence of microsatellite repeats does not capture the essence of microsatellite instability, as it focuses on their lack, rather than their fluctuating presence. Similarly, stating the correct number of microsatellite repeats fails to reflect the very nature of instability, which inherently involves variation. Lastly, the presence of only short tandem repeats simplifies the context too much, ignoring the complexities of the variations in length that define microsatellite instability. Here, the emphasis is on how instability results in incorrect repetitions rather than merely focusing on short versus long repeats.