Tyrosine-protein kinase transmembrane receptor ROR1, also known as neurotrophic tyrosine kinase, receptor-related 1 (NTRKR1), is an enzyme that in humans is encoded by the ROR1 gene. ROR1 is a member of the receptor tyrosine kinase-like orphan receptor (ROR) family. The protein encoded by this gene is a receptor tyrosine kinase that modulates neurite growth in the central nervous system. It is a type I membrane protein and belongs to the ROR subfamily of cell surface receptors. The expression of soluble ROR1 does not differ between normal individuals and tumor patients, and the expression level in serum is low or undetectable, independent of disease progression and severity. However, intact membrane bound ROR1 is specifically expressed at high levels in various tumor tissues. ROR1 is currently under investigation for its role in the metastasis of cancer cells. ROR1 has been found in tumor cells of many different types of cancer, such as chronic lymphocytic leukemia (CLL) and other hematological malignancies. It appears that ROR1 is mainly expressed in poorly differentiated tumors with high potential for recurrence and metastasis.