Abstract Neurofibromatosis type 1 (NF1) is an autosomal dominant neurocutaneous disorder. A part of the gene for NF1 was cloned and its deduced protein has a domain functionally related to mammalian ras GTPase-activating protein (GAP). Human tissues examined express two types of NF1 mRNAs: an originally identified species of NF1 mRNA (type I) and another one containing the 63 base insert in the region coding for GAP-related domain (type II). However, relative levels of both mRNAs seem to change under certain conditions. Human brain expresses type I mRNA predominantly, while type II is preferentially expressed in most primary brain tumors ( 13 16 tumors analyzed.). We suggest that higher levels of type II mRNA may be related to the genesis of brain tumors.