They demonstrated that ITK-SYKCD4-creERT2 mice with homozygous/heterozygous PDCD1 deficiency were affected by the severe invasion of ITK-SYK+PD-1−CD4+ T-cells into the solid organs, indicating that PD-1 is a haploinsufficient tumor suppressor. The gene discussed is PDCD1; the disease is neoplasm.