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We have developed and validated a highly specific, versatile antibody to the extracellular domain of human LGR5 (α-LGR5). α-LGR5 detects LGR5 overexpression in >90% of colorectal cancer (CRC), hepatocellular carcinoma (HCC) and pre-B-ALL tumour cells and was used to generate an Antibody-Drug Conjugate (α-LGR5-ADC), Bispecific T-cell Engager (α-LGR5-BiTE) and Chimeric Antigen Receptor (α-LGR5-CAR). α-LGR5-ADC was the most effective modality for targeting LGR5+ cancer cells in vitro and demonstrated potent anti-tumour efficacy in a murine model of human NALM6 pre-B-ALL driving tumour attrition to less than 1% of control treatment. α-LGR5-BiTE treatment was less effective in the pre-B-ALL cancer model yet promoted a twofold reduction in tumour burden. α-LGR5-CAR-T cells also showed specific and potent LGR5+ cancer cell killing in vitro and effective tumour targeting with a fourfold decrease in pre-B-ALL tumour burden relative to controls. Taken together, we show that α-LGR5 can not only be used as a research tool and a biomarker but also provides a versatile building block for a highly effective immune therapeutic portfolio targeting a range of LGR5-expressing cancer cells. Synopsis: Highly specific antibodies against the extracellular domain of LGR5 were developed as (i) a research tool to study LGR5 biology, (ii) for diagnostic use in multiple cancer types and (iii) as novel immunotherapeutics. A unique antibody toolkit has been validated for high affinity and specific detection of human LGR5. α-LGR5 was established as a diagnostic for discriminating high LGR5 expressing colorectal cancer (CRC), hepatocellular carcinoma (HCC) and pre-B-ALL tumours relative to low expression in healthy human tissues. α-LGR5-derived Antibody Drug Conjugates (ADCs), Bispecific T cell Engagers (BiTEs) and Chimeric Antigen Receptor (CAR) T cells were developed and displayed in vivo efficacy in a pre-B-ALL murine tumour model. Highly specific antibodies against the extracellular domain of LGR5 were developed as (i) a research tool to study LGR5 biology, (ii) for diagnostic use in multiple cancer types and (iii) as novel immunotherapeutics. [ABSTRACT FROM AUTHOR]