In an experiment discussed in class, Francis Crick isolated revertants of the mutation FC0. FC0 is a ( ) frameshift mutation in the rIIA cistron of phage T4. Most of the isolated revertants resulted from second site mutations within the rIIA cistron. One of these second site mutations was called FC10. Crick then separated away FC10 from FC0 and found that FC10 had an rII- phenotype. He then selected for phenotypic revertants of the FC10 mutation and recovered a second site suppressing mutation that he called FC63. Is FC63 a ( ) frameshift mutation or a (-) frameshift mutation.