Parents please take note. The frequency of brain tumours has been underestimated in children with the common genetic syndrome -- neurofibromatosis type 1 (NF1) a new study has found. According to the researchers this disorder is characterised by birthmarks on the skin and benign nerve tumours that develop in or on the skin. Brain tumours are also known to occur in children and adults with NF1. They estimated that only 15-20 per cent of kids with NF1 develop brain tumours. But the study published in the journal Neurology: Clinical Practice found that the frequency of brain tumours in this population