The present invention provides an isolated polynucleotide comprising at least one polymorphic nucleotide sequence, for example, at position -871 from the promoter region of the BLyS gene, and diagnostic assays for detecting the presence of this polymorphism associated with a condition associated with BLyS activity, such as hematological malignancy including B cell malignancies. The diagnostic assays are useful in predicting an individual's likelihood of developing a condition associated with BLyS activity, such as hematological malignancies, and for methods for treating an individual clinically diagnosed with a condition associated with BLyS activity, such as prediction of a patient's likelihood to respond to a specific drug treatment. The invention also provides an array of nucleic acid molecules immobilized on a solid surface, where at least one of the nucleic acid molecules comprises a BLyS polymorphic nucleic acid molecule. The nucleic acid arrays of the invention allow rapid detection of hybridizing nucleic acid-molecules, in a nucleic acid sample from an individual, of a BLyS polymorphism associated with hematological malignancy.