Increased expression of resistance sequences is associated with drug resistance of certain cells (e.g., cancer cells). The invention provides methods for identifying drug resistant cells by measuring the expression or activity of resistance genes (e.g., semaphorin D, B94, mel-14 antigen, 24p3, proliferin, or maspin), methods for identifying modulators of drug resistance, and methods for modulating drug resistance by modulating the expression or activity of resistance sequences.