Being abstracted from BioGPS now, MyGene.info provides simple-to-use REST web services to query/retrieve gene annotation data. It's designed with simplicity and performance emphasized. Check out MyGene.info website or click the links below for the detailed API documention:
Gene Query API at MyGene.info
For developers, BioGPS provides extensive REST (REpresentational State Transfer) based web service APIs for programmatic access of BioGPS resources. Click the links below for the detailed API documentation: