S912L means the amino acid, serine, at postion 912 was converted to the amino acid leucine. This mutation is generally considered neutral unless it is inherited cis (meaning on the same chromosome) as another CFTR mutation. This means that in order for you to have CF (as opposed to being a carrier) you would need to have S912L and another mutation on the same chromosome AND yet another CFTR mutation on the trans or the "other" chromsome.
A study was conducted in tissue culture cells where they compared your mutation to a mutant chromosome containing both your mutation (s912L) and another mutation (g1244V). The activity the Cl- channel was not affected in the cells that had ONLY s912l but was decreased in the cells that had both mutations on the same chromosome.
here is a link:
www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=15744523&query_hl=1&itool=pubmed_docsum