When an animal egg is fertilized, the zygote rapidly transforms the extracellular matrix (ECM) of the egg into a protective structure in which embryogenesis can proceed. On p. 431, Wong and Wessel provide new insights into the biochemistry of this important transformation in the sea urchin fertilization envelope. Soon after insemination, they report, the fertilization envelope becomes impermeable to molecules larger than 40,000 daltons (medium-sized proteins;for example, serum albumin) via a peroxidase-dependent protein crosslinking mechanism. Using a new in vivo technique to label and isolate the modified ECM components, they show that four major components of the fertilization envelope are selectively crosslinked in a manner that distinguishes between protein isoforms derived from the same gene. The authors speculate that this selectivity is partly due to the local clustering of target proteins within the fertilization envelope and conclude that the free-radical crosslinking of specific proteins is essential for establishing the embryonic microenvironment that is needed for early development.