Sir—Hartmut Koch (Dec 5, p 1824) 1 rightly draws attention to two points about correct diagnosis of molybdenum cofactor deficiency: first, the unreliability of a single sulphite dipstick test in alerting medical or laboratory staff to the need for further investigations of purine metabolism; and second, the importance of uric acid measurement for neonatal seizures. However, there are several additional difficulties for the attending physicians or investigating laboratories which might obscure diagnosis. Not all cases present neonatally with a virtual absence of uric acid in plasma and urine, the classic hallmark of this disorder. 2 Although this observation applies to most early presenters, phenotypic variation in expression of the defect is now evident. Late presenters can have plasma uric acid at the bottom end of the normal range for children. Since children have a low plasma uric acid compared with adults because of a high renal clearance, diagnosis may well be missed, or delayed. 3 For example, an 8-year-old otherwise healthy sibling attending Moorfields Eye Hospital for dislocated lenses was identified only during family screening when a younger sister with neurological problems developing at 12 months was diagnosed at age 2 years from a persistently low plasma urate, all other tests having proved negative. 3 Confirmation of the cofactor deficiency should be made in either instance, or in any patient with a positive dipstick test (which could also indicate isolated sulphite oxidase