The tissue distribution of deoxyribonuclease 1 (DNASE1, DNase I), a Ca2+ and Mg2+/Mn2+-dependent secretory endonuclease, has previously been investigated. However, most of these studies did not account for the existence of different members of the DNASE1 gene family, did not differentiate between endogenous DNASE1 protein synthesis and its extracellular occurrence or were not performed with methods allowing both a sensitive and a specific detection. Now we re-examined the DNASE1 gene expression pattern by taking advantage of the Dnase1 knockout mouse model. Direct comparison of samples derived from wild-type (Dnase1+/+) and knockout (Dnase1-/-) mice allowed an unambiguous detection of Dnase1 gene expression at the mRNA and protein level. For the detection of Dnase1 activity, we developed a highly sensitive nuclease zymogram method. We observed high Dnase1 gene expression in the parotid and submandibular gland as well as in the kidney and duodenum, intermediate expression in the ileum, mesenterial lymph nodes, liver, ventral prostate, epididymis, ovary and stomach, and low expression in the sublingual, preputial, coagulation and pituitary gland. We report for the first time the lachrymal and thyroid glands, the urinary bladder and the eye to be Dnase1-expressing organs as well. Since Dnase1 knockout mice with the 129xC57Bl/6 mixed genetic background have indicated the protection against an anti-DNA autoimmune response as a new physiological function of Dnase1, knowledge of the physiological sites of its synthesis might prove helpful to find new therapeutic strategies.