def _psc1(psc1, psc2_from_psc1):
    if 'TEST' in psc1.upper():
        # skip test subjects
        logging.debug('skipping test subject "%s"', psc1)
    else:
        # find and skip subjects with invalid identifier
        if psc1[-3:] in {'FU2', 'FU3'}:
            psc1 = psc1[:-3]
        elif psc1[-2:] == 'SB':
            psc1 = psc1[:-2]
        if psc1 in psc2_from_psc1:
            return psc1
        elif psc1 in {'0x0000xxxxxx'}:
            logging.info('skipping known invalid subject identifier "%s"',
                         psc1)
        else:
            logging.error('invalid subject identifier "%s"', psc1)
    return None