We suggest that this may be because the TPH2 KI mice, due to their reduced serotonin levels, already show anxiety- and depressive-like behaviors, and reduced serotonin levels at baseline (Zhang et al., 2005; Beaulieu et al., 2008; Jacobsen et al., 2012b), such that it is physiologically not possible to show exacerbated anxiety- or depressive-like phenotype in response to MS. The gene discussed is TPH2; the disease is myeloid sarcoma.