Contamination of human DNA samples with mouse DNA can lead to false detection of XMRV-like sequences.

Observed Findings

• Nested PCR assays generated positive XMRV/MLV results in multiple samples from both patients and controls, but specific TaqMan qPCR detected no XMRV in any sample.
• All samples testing positive for XMRV/MLV DNA were also positive for murine IAP long terminal repeat sequences.
• Most positive samples contained murine mitochondrial cytochrome oxidase DNA sequences, indicating mouse cell contamination.
• DNA sequencing of positive PCR products revealed a wide variety of sequences, some matching previous patient findings and others matching known endogenous MLVs.
• No contamination was detected in negative control samples lacking DNA template.

Inferred Conclusions

• Mouse DNA contamination, rather than true XMRV infection, explains positive XMRV/MLV results in this cohort of ME/CFS patients and healthy controls.
• Even trace amounts of mouse DNA (much less than one cell's worth) can produce detectable PCR products using highly sensitive nested PCR assays.
• Specific, sensitive assays and monitoring for mouse DNA contamination are essential for accurate detection of retroviruses in clinical samples.
• Methodological rigor in assay design is critical for distinguishing true infections from contamination artifacts in viral detection studies.

Remaining Questions