What is the target with an approval date of 2006?
CREATE TABLE "cancer_immunotherapy_monoclonal_antibodi" (
  "antibody" text,
  "brand_name" text,
  "approval_date" real,
  "type" text,
  "target" text,
  "approved_treatment_s" text
);
SELECT "target" FROM "cancer_immunotherapy_monoclonal_antibodi" WHERE "approval_date"=2006;