Greek letters are used in many ways in science, usually as an alternative to the Roman alphabet. Some of these ways are:

• The symbol for "micro" as a prefix to scientific units, since "m" was already used for "milli" and "M" for "Mega", the greek m was used, as in mm for micrometers. Other scale factors for scientific units (such as meters, seconds, bytes) all use roman letters, including:
  yocto (10^-24) y, zepto (10^-21) z, atto (10^-18) a, femto (10^-15) f, pico (10^-12) p, nano (10^-9) n, micro (10^-6) m, milli (10^-3) m, centi (10^-2) c, deci (10^-1) d, deka (10¹) da or D, hecto (10²) h, kilo (10³) k or K, mega (10⁶) M, giga (10⁹) G, tera (10¹²) T, peta (10¹⁵) P, exa (10¹⁸) E, zetta (10²¹) Z, yotta (10²⁴) Y.

• Electrophoretic mobility: bands are called a, b, g based on the distances they migrate in the electrophoresis gel. Examples: a-, b-, and g-globulins.

• Chains making up the quaternary structure of a protein molecule are typically called a, b, g etc. Usually the chain called a is the largest, with decreasing sizes for chains given subsequent letters in the alphabet. Examples: hemoglobin is a tetramer with two a chains and two b chains (an a₂b₂ molecule). Antibody, one of the first proteins to have its chains identified, is exceptional in having its chains called simply "heavy" and "light". The TCR is associated with a complex designated CD3, composed of gde₂z₂. Note that the TCR gd chains are different from the gd chains of CD3.

• Different forms of a molecule. T lymphocyte antigen receptors come in two forms, ab and gd. Interferons come in three forms, a, b and g.