Genzyme Begins Trial of Myozyme® for Patients with Late-Onset Pompe Disease

Date: September 15, 2005

Genzyme Corp. (Nasdaq: GENZ) announced today that it has initiated a clinical trial evaluating the safety and efficacy of Myozyme® (alglucosidase alfa) in patients with late-onset Pompe disease. The trial is designed to provide additional support for Myozyme's use.

Because a treatment for Pompe disease is urgently needed, Genzyme has already submitted European and U.S. marketing applications for Myozyme. Regulatory action on these applications is expected in early 2006. Genzyme anticipates submitting marketing applications for Myozyme in Japan and several other countries by the end of this year.

If approved, Myozyme will be the first treatment developed for patients with Pompe disease, a debilitating and often fatal neuromuscular disorder affecting fewer than 10,000 people worldwide. Patients with Pompe disease display a range of symptoms that correspond with the age of disease onset and the rate of disease progression. Progressive weakness in the muscles used for mobility, breathing and digestion are the most common symptoms. In infants, heart muscle is typically severely affected.

The late-onset trial is a randomized, double-blind, placebo-controlled study that will enroll at least 72 patients. It is being conducted at five sites in the United States and two in Europe. Participants will be assigned to receive intravenous infusions of either Myozyme at a dose of 20 mg/kg or a placebo every other week for 52 weeks. Approximately two-thirds of participants will receive Myozyme. The study has two primary efficacy endpoints: (1) to determine the effect of Myozyme on functional endurance as measured by the Six-Minute Walk Test; and (2) to determine the effect of Myozyme on respiratory muscle weakness as measured by Forced Vital Capacity. These endpoints are the same as those used in the pivotal clinical trial of Aldurazyme® (laronidase), which was approved for the treatment of MPS I disease in 2003.

"Conducting this study is consistent with our commitment to confirm the safety and efficacy of Myozyme for patients across the spectrum of Pompe disease," said Richard A. Moscicki, senior vice president and chief medical officer for Genzyme. "Our approach has been to pursue approval for Myozyme as soon as possible, given the tremendous medical need. At the same time, we have worked to put in place a broad, supportive program that includes natural history studies, a disease registry, an expanded access program and additional clinical research."

Genzyme recently completed an observational study involving approximately 60 patients with late-onset Pompe disease. The study was designed to evaluate endpoints for the treatment study. Genzyme anticipates that many of the patients from the observational study may meet the entry criteria for the treatment study.

About Pompe disease

Pompe disease is one of more than 40 genetic diseases called lysosomal storage disorders, which are caused by a deficiency or malfunction of specific enzymes found in cell lysosomes. Pompe disease manifests as a broad spectrum of clinical symptoms with varying rates of disease progression, typically divided into two broad classifications:

Infantile-onset Pompe disease:

Babies who present with symptoms in the first few months of life are typically considered to have this form of Pompe disease. Without treatment, an infant may become seriously ill and die from cardiac or respiratory complications within the first year of life. Common symptoms of infantile-onset Pompe disease include severe and rapidly progressive muscle weakness, enlarged heart, respiratory insufficiency and frequent respiratory infections, cardiac complications, delayed development and feeding problems. About one-third of people with Pompe disease have infantile-onset Pompe.

Late-onset Pompe disease:

Late-onset patients may present with muscle or respiratory weakness anytime during childhood or adulthood, and disease progression is less rapid but steady. These patients will experience a shortened life-span due to progressive respiratory failure and often require mechanical ventilation to assist with breathing as well as mobility aids such as canes, walkers or wheelchairs. Common symptoms of late-onset Pompe disease include progressive muscle weakness in the legs and hips; gradual loss of ability to walk, run or jump; respiratory problems; morning headaches; scoliosis; daytime sleepiness; and difficulty gaining or maintaining weight. About two-thirds of people with Pompe disease have the late-onset form of the disease.

About Genzyme

One of the world's leading biotechnology companies, Genzyme is dedicated to making a major positive impact on the lives of people with serious diseases. Founded in 1981, Genzyme has grown from a small start-up to a diversified enterprise with 2004 revenues of $2.2 billion and more than 7,600 employees in locations spanning the globe. With many established products and services helping patients in more than 80 countries, Genzyme is a leader in the effort to develop and apply the most advanced technologies in the life sciences. The company's products and services are focused on rare inherited disorders, kidney disease, orthopaedics, cancer, transplant and immune diseases, and diagnostic testing. Genzyme's commitment to innovation continues today with a substantial development program focused on these fields as well as heart disease and other areas of unmet medical need.

This press release contains forward-looking statements, including statements about clinical trial and regulatory plans and timelines for Myozyme, including the design of the clinical trial for late-onset Pompe patients, the anticipated timing of action by US and EU regulatory authorities on marketing applications for Myozyme, the submission of marketing applications in Japan and other countries and the expected timing thereof, and the number of patients involved in the observational study that are expected to meet the eligibility criteria for the late-onset trial and the anticipated impact on the enrollment rate for the late-onset trial; the anticipated commercial availability of Myozyme; and Pompe disease patient population estimates.

These statements are subject to risks and uncertainties that could cause actual results to differ materially from those projected in these forward-looking statements.

These risks and uncertainties include the actual enrollment rate for the clinical trial for late-onset Pompe patients and the actual timing of and safety and efficacy results from this trial, the actual timing and content of submissions to and decisions made by the EU, US, Japanese and other regulatory authorities regarding the marketing authorization applications for Myozyme and labeling for Myozyme, the classifications of Pompe disease covered by any approved labeling for Myozyme, the timely receipt of pricing and reimbursement approvals in approved countries, the ability to manufacture sufficient quantities of product for development and commercialization activities and to do so in a timely and cost efficient manner, the accuracy of Genzyme's estimates concerning the Pompe disease patient population, and the risks and uncertainties described in reports filed by Genzyme with the Securities and Exchange Commission.

Please see the disclosure under the heading "Factors Affecting Future Operating Results" in the Management's Discussion and Analysis of Financial Condition and Results of Operations section of Genzyme's Quarterly Report on Form 10-Q for the period ended June 30, 2005 for a more complete discussion of these and other risks. Genzyme cautions investors not to place substantial reliance on the forward-looking statements contained in this press release.

These statements speak only as of the date of this press release, and Genzyme undertakes no obligation to update or revise the statements.

Genzyme® and Myozyme® are registered trademarks of Genzyme Corporation, and Aldurazyme® is a registered trademark of BioMarin/Genzyme LLC. All rights reserved.