 As queuine appears to be essential for the maintenance of adequate BH4 levels, queuine deficiency—resulting from disruption to the microbiome by factors like antibiotics—has been proposed as a possible cause of mental illnesses related to imbalances of these neurotransmitters.Although general queuine repletion status has not been studied in humans, studies of cancerous tissue have found a uniform trend of queuine deficiency in lung, ovarian, and lymphatic cancers.In one study of patients with lung cancer, a lack of queuosine in tumor tissue tRNA was associated with worse odds of survival four years post-biopsy.