Among the alternative splicing isoforms in organism transcriptome, how many are functional is still disputable. Some alternative splicing may cause premature termination codon (PTC); and the alternative isoforms with PTC can be potentially targeted for degradation by the nonsense mediated mRNA decay (NMD) surveillance machinery. According to ASPicDB, the number of transcripts of the human genome is about 320,000, among which only 30,000 may generate PTC+ isoforms. If this part is discarded, there will be no important effect on the calculation. As PTC related data of most organisms are still unknown, this part is not taken into consideration at this time.