Motivation: Sanger sequencing is still being employed for sequence variant detection by many laboratories, especially in a clinical setting. However, chromatogram interpretation often requires manual inspection and in some cases, considerable expertise. Results: We present GLASS, a web-based Sanger sequence trace viewer, editor, aligner and variant caller, built to assist with the assessment of variations in 'curated' or user-provided genes. Critically, it produces a standardized variant output as recommended by the Human Genome Variation Society.