Failure in response to FMT treatment may be due to various reasons including the inability to replace the host microbiota and successfully implant beneficial microbiota favoring anti-PD-1 treatment into the recipient, the inability to respond to the tumor progression regardless of beneficial microbiota obtained because of the patient’s own immunodeficient status or lack of tumor immunogenicity, or a total absence of microbiota needed for anti–PD-1 therapy effectiveness in the FMT provided to them [42]. This evidence concerns the gene PDCD1 and neoplasm.