The neonatal splice variant of the sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA1b) is expressed specifically in myotubes and developing skeletal muscle1. This pump has a functional difference from its adult isoform, SERCA1a. The SERCA1a is expressed in myofibres of fast twitch muscle and has more capacity to pump against high vesicular Ca2+ concentration than SERCA1b does2. The only structural variance – the octapeptide tail in SERCA1b instead of the C-terminal glycine of SERCA1a3 – does not seem to explain the functional difference because the C-terminal tail protrudes into the sarcoplasm, not the lumen of the sarcoplasmic reticulum where the concentration of Ca2+ is accumulated4. This paradox hints to a possible regulatory mechanism exerted by micro peptides recently found abundant in developing muscle5. A developmental effect of this possible regulation will be presented.