It is a vector with names = probe set names and values = fold changes,
with "-1" depicting probe sets whose target RNAs were not spiked in ("empty"),
and "-2" marking "mixed" probe sets. In summary,
1: S/C=1, 2535;
-1: "empties", 10131;
-2: "mixed", ambiguous, 13, excluded;
>1: 1331 nonnulls.